In the U.S. criminal code, specifically 8 USC 1325, it says illegal aliens "shall, for the first commission of any such offense, be fined under Title 18 or imprisoned not more than six months, or both, and, for a subsequent commission of any such offense, be fined under Title 18, or imprisoned not more than two years, or both."
President Bush did not enforce this law as he swore to do when inaugurated and Obama has not and will not either. Here is a recent comment of Obama's Secretary of Home Land Security.
During a recent interview on CNN's "State of the Union With John King," Homeland Security Secretary Janet Napolitano was asked what she thought about the statement by Sheriff Joe Arpaio of Maricopa County, Ariz., that the feds should prosecute all caught crossing the border illegally. She responded by blaming employers, "the human traffickers who are exploiting human misery," and said that "when we find illegal workers, yes, appropriate action, some of which is criminal, most of that is civil, because crossing the border is not a crime per se. It is civil."
So the federal government ignores the law and turns on Sheriff Arpaio who just wants to enforce that which the government put into law.