President Obama’s new set of immigration policies could affect as many as 5 million people, including the possibility of a three-year reprieve from the threat of deportation for parents of children with legal status.

The new year will see those policies coming into effect, potentially creating dramatic changes for those who are in the U.S. illegally. Also ahead in 2015 are important shifts in how agents will enforce immigration laws to focus more on deporting people with lengthy or violent criminal records and less on people whose only crimes are immigration offenses.

The new approach will end the dragnet system that enlisted police in blowing the whistle on immigrants. These policies won’t apply to most of the 11.2 million living in the country illegally.

And don’t expect this to roll out without a fight. Republicans in Congress already have vowed to try to undo the new policies.

“This is a serious breach of our Constitution. It’s a serious threat to our system of government,” House Speaker John A. Boehner said as the plan was unveiled. But practically speaking, there is little they can do.

Republican governors in states affected by the new deportation policies have called out the lawyers. At least 24 states have filed suit to block the plan, and that case is expected to play out in the courts throughout 2015.

Here’s a look at the plan and what we can expect:

Whom is the new plan designed to help?

Mostly immigrants who came to the U.S. illegally and who have children who are citizens (nearly all children born in the U.S. are automatically citizens) or permanent legal residents. To be eligible, people have to have been living in the U.S. since Jan. 1, 2010, and have no record of serious crimes that would make them a priority for removal.

A White House legal memo said this “would serve an important humanitarian interest in keeping parents together with children who are lawfully present in the United States.” Approved applicants will get permission to stay for three years. As many as 4.1 million people could fit the criteria, the administration estimates. The application will cost $465.

What about people who came to the U.S. as children?