What is DHS defining as "criminal?"

Newspapers are reporting today that new DHS Secretary Janet Napolitano wants "alien criminals" out of the U.S.

"Homeland secretary wants criminal aliens out of US" (Washington Post/AP, January 29, 2009)  is stating:

"About 113,000 criminals who were in the U.S. illegally were deported last year, Immigration and Customs Enforcement said. The agency estimates there are now as many as 450,000 criminals in federal, state and local detention centers who are in the country illegally."

Are they considered criminals for having been in the U.S. without a visa?  Or not reporting for an ICE summons, or for murdering people?   Considering that so many studies of immigrant populations have found that the actual criminal rate among immigrants is extremely low, I am wondering what the criteria is for the latest need to eject people.