1. Federal judge Andrew Hanen ruled on Monday night that the Obama administration has to temporarily halt implementation of its executive actions on immigration.
  2. As a result, DHS Secretary Jeh Johnson announced Tuesday that the Obama administration won't be accepting applications for protection from deportation and work permits from immigrants older than 30 who came the US as children. It was supposed to start accepting those applications Wednesday.
  3. The administration has also put on hold a program that was supposed to open later this spring, for parents of US citizens and permanent residents.
  4. The injunction means Hanen thinks there's a substantial chance that he's going to ultimately rule that the executive actions were made illegally, siding with the 26 states that have sued the administration.
  5. The Obama administration is expected to file to keep the injunction itself from going into effect; the Fifth Circuit will take up the question in a few weeks.

Relief is delayed for millions of people

Here's what this means: Until this ruling is reversed or a different ruling comes down in the future, the federal government isn't allowed to do anything to implement either of the new programs President Obama announced in November to protect unauthorized immigrants from deportation.
Between the two programs, millions of immigrants were supposed to be eligible for deferred action (three years of protection from deportation) and work permits. Neither of those programs had actually started accepting applications yet, although one was supposed to start on Wednesday. Now they won't be able to start until further notice.
Immigrants who are older than 30 but who came to the US as children or teenagers (and meet other requirements) were supposed to be able to apply for deferred action starting on February 18, an expansion of the existing Deferred Action for Childhood Arrivals program. Now they'll have to wait. That's about 290,000 immigrants who'll remain vulnerable to deportation.
There are also the 3.7 million unauthorized immigrants who are parents of US citizens or permanent residents who'd benefit from another program: Deferred Action for Parents of Americans. Applications were supposed to open this spring, but the government still hasn't put out the official application requirements and fee; it won't be able to finalize that, or announce it, under this ruling.
Why this is a serious threat to Obama's immigration policy

There's going to be continued legal wrangling over whether or not Hanen's ruling is upheld, and what the final outcome of the case is (see below for more on that). But even if this ruling is quickly overturned, and the administration is allowed to move forward with granting deferred action, the success of the relief programs might be in jeopardy.
When the administration created the first deferred-action program in 2012, for young unauthorized immigrants, they discovered the success of the program relied on people signing up — and on the ground, organizers learned that finding eligible immigrants and getting them to apply was the hardest part. Now, community groups are trying to educate a much larger, more diffuse immigrant population about the new deferred-action programs, and persuade them that it's safe to apply. But news and misinformation about the lawsuit is spreading confusion and fear among the very people these groups are trying to reach.
Organizers are worried about a "chilling effect": by the time applications do open for deferred action, immigrants will have been intimidated out of applying, because they won't believe the program is safe or permanent.
Does this mean Obama's executive actions have been found unconstitutional?

No. The reason that Judge Hanen is stopping Obama's actions actually isn't about the Constitution at all. As Josh Blackman, a professor at the South Texas College of Law who filed a brief on behalf of the states in this case, says, "Generally, courts will not reach the constitutional question unless they need to."
Instead, Hanen's ruling is about a procedural law called the Administrative Procedures Act, and in particular the "notice and comment requirement" — which is the typical procedure for making federal regulations. According to Cecilia Wang, Director of the Immigrant Rights Project for the ACLU, Hanen's ruling says that "if (the government) wanted to do these things it should have provided notice in the Federal Register, with period for comment." But because the Obama administration didn't do that for these actions, the ruling says, it violated the law.
Furthermore, this isn't even Judge Hanen's final opinion on the matter. This ruling is an injunction: it means that while Judge Hanen hasn't decided whether or not the president's executive actions are unconstitutional or illegal, the government has to stop acting on them while the judge makes up his mind.
In this type of case, the judge only issues an injunction if he thinks there's a good chance he'll side against the government when he makes a final ruling. So it's definitely not good news for the administration. But it's still possible that he'll ultimately decide that Obama's actions were kosher.
It's also possible, according to Wang (though Blackman disagrees), that the Obama administration could call Judge Hanen's bluff, and simply start going through the typical regulation process, including the "notice and comment" steps, to validate the executive actions. That would answer the main objection Hanen's ruling made.
Whom, exactly, does the injunction affect?

The injunction says that the federal government has to stop both of its new efforts to grant deferred action (temporary protection from deportation) and work permits to unauthorized immigrants. So it can't start accepting deferred-action applications from unauthorized immigrants who are older than 30 but who came to the US when they were children or teenagers, as it was planning to do on February 18th. Nor can it put out and start accepting applications for deferred action from unauthorized immigrants whose children are US citizens or permanent residents, as it was planning to do later this spring.
However, immigrants who are currently under 30, and who came to the US as children are teenagers, are still able to apply for deferred action. That's because the lawsuit isn't challenging the original Deferred Action for Childhood Arrivals program that was implemented in 2012. It's just challenging the expansion of that program and the separate program for parents, both announced in 2014.
What are the states suing the administration?

26 states have joined the lawsuit. (Most are suing directly as the state, via the attorney general; in a few cases where the attorney general is a Democrat but the governor is a Republican, the governor joined the lawsuit on his own.) On the other hand, 13 states filed a brief siding with the federal government and urging the judge to let the actions stand. Here's the breakdown:

Of course, since Republican states are (by and large) less diverse than Democratic ones, this means that the states suing the Obama administration over immigration aren't the states with the most immigrants. In total, the states suing the president have about 2 million unauthorized immigrants who could benefit from deferred action, or about 38 percent of all immigrants who could benefit (according to the Migration Policy Institute). Without Texas, that drops to 24 percent. The 13 states (plus the District of Columbia) siding with the administration have 2.6 million potentially eligible immigrants — half of all immigrants who could benefit.
And in addition to the states that have sided with the federal government, 30 cities have filed on Obama's side as well — including some cities in states that are on the opposite side of the suit, like Houston.
How can states even sue to stop the federal government over this?

This is actually part of the lawsuit itself. The federal government is arguing that the states don't even have the authority (called "standing") to bring this lawsuit into court at all — because they can't show that the state governments themselves are being harmed by immigrants getting deferred action.
The states are arguing that they have to "bear the burden" of unauthorized migration by paying for public schooling for unauthorized immigrant children; including US citizen children whose parents are unauthorized in safety-net programs; and especially, by having to provide "uncompensated" emergency care to unauthorized immigrants who don't have health insurance, since they're barred from participating in the exchanges under Obamacare and don't have low-cost insurance options. Furthermore, they claim, Obama's executive actions will encourage more unauthorized immigrants to come to the US, which will burden the states even more.
Hanen didn't even take a close look at most of those claims — or he rejected them outright. Instead, the main reason he said the states had standing is that Texas — one of the 26 states — would have to spend millions of dollars on drivers' licenses for deferred-action recipients. So just like Hanen used regulatory procedure as the main reason to stop the programs from going into effect, he used a very narrow justification for the states' standing.
What happens next?

At this point, it's up to the Department of Justice to decide whether they want to ask for a stay — basically, something to enjoin the injunction. They're (obviously) expected to do that. Then it'll be up to the Fifth Circuit to consider the stay. If they side with the federal government, the program will start running again; if they shoot down the federal government again, implementation will have to wait.
The ruling on the stay will determine whether or not the president's policies move forward for the next several months. During that time, the Fifth Circuit will be considering whether to uphold Judge Hanen's injunction. That could take four to five months, if they fast track the case, or seven to eight months if they don't. At the end of that time, they'll have another opportunity to stop the program or restart it: even if they grant the government a stay this winter, if they uphold Judge Hanen's ruling later in the year, the government has to stop again.
Meanwhile, Judge Hanen himself still has to issue his final ruling on whether or not the executive actions were constitutional.
When will the ruling on the stay happen?

The federal government has been preparing to open deferred-action applications up to older immigrants who arrived as kids or teens on February 18 — this Wednesday. That could put the Fifth Circuit on a tight deadline: they might want to rule on the stay as close to Wednesday as possible, so that the federal government can either stay on schedule or know it'll be set back for several more months. Typically, however, it takes a few weeks for courts to rule on emergency stays in cases like this.
Is this a bad sign for the ultimate survival of the president's actions in court?

Judge Hanen isn't necessarily the best predictor of how a higher court might rule. The group of states suing the president deliberately chose to file their suit in this district, because it had the best chance of going their way (defendants of the president's policy call this "shopping" for a favorable judge). And supporters of the administration were displeased when Hanen got selected to hear the case. Hanen has made some pretty hard-line rulings about immigration in the past. In 2013, after convicting a defendant for smuggling a 10-year-old girl into the US, Hanen issued an order attacking the US government for allowing the girl to reunite with her mother who was in the US — and said the government should have brought criminal charges against the mother instead.
Some legal advocates filing with the president believe that the higher the case goes in the court system, the more likely judges are to side with the administration. That's especially true because, despite the political charge that immigration carries, the legal question here is about the power of the executive branch — and some conservative judges believe in a strong executive. But it also depends on which judges get randomly picked to rule on the case at the next stage.
In December, Marshall Fitz of the Center for American Progress was optimistic: he said there would be "a lot of drama," but "we're going to win in the courts, eventually." Now, though, advocates are more concerned about how long it's going to take to get to that "eventually."

CORRECTION: This article originally referred to a possibility that the 5th Circuit might rule on a stay before February 18. That is exceedingly unlikely, given the timeframe. The article's been tweaked to reflect a more reasonable timeline.
The article also previously said that the ACLU filed a brief supporting the Obama administration.
UPDATE: This article has been updated with details regarding the grounds of the ruling, and analysis from legal scholars on both sides of the case.