Under federal law, it is a crime for anyone to enter into the US without the approval of an immigration officer -- it's a misdemeanor offense that carries fines and no more than six months in prison. Many foreign nationals, however, enter the country legally every day on valid work or travel visas, and end up overstaying for a variety of reasons. But that's not a violation of federal criminal law -- it's a civil violation that gets handled in immigration court proceedings.
Source: wiktionary