Many schools use the AAMC's background check, which is through a service called Certiphi. It checks the local courts for every address you've lived, state and federal courts, for misdemeanors and felonies. It also checks the sex offender registry and I believe immigration/customs (INS) as well. Schools that don't use the background check through Certiphi usually have their own, which could be either more or less intense than the AAMC check. You can find a list of the schools' policies on the AAMC website.