This is a good summary:
There’s one fundamental difference between the new White House and the old when it comes to immigration: Barack Obama ordered his administration not to enforce a number of immigration laws. Donald Trump has ordered his administration to enforce them.
Trump’s two immigration executive orders, issued Wednesday, are long, far-reaching, and complicated. But perhaps the most consequential passage in the two combined orders is a single sentence: “The purpose of this order is to direct executive departments and agencies to employ all lawful means to enforce the immigration laws of the United States.”