I have been researching and testing background check services for over 2 years for a consumer review site (http://www.CompleteReviews.net ). Employment screening background checks can cover a wide range of information. Here is a list of what can be included on one of these reports. Bear in mind that not every employer will elect to include every one of these search categories in their background checks but you can at least see what they potentially could find.

Social Security Number (SSN) Trace

Address History Search

Multi-State Criminal Database Search

County Criminal History Search

Federal Criminal History Search

Sex Offender Registry Search

Employment Verification Search

Education Verification

Credential Verification

Reference Checks

Financial Sanctions Search

Healthcare Sanctions Searches

OFAC/Patriot Act Search

Global Blacklist Search

Nations Security Council, World Bank and other foreign governments.

Motor Vehicle Record (MVR) Check

Credit Check

They are running a criminal background check to see if you have a record, also some places may run a credit check.

It depends on what type of background checks they do...criminal, credit, previous employment, reference checks, educational, etc

I've never known an employer to run a background check until they decided to hire you. At my current job we do a criminal record check, and for some positions we do a sex offender record check as well. Some employers also do a credit check. We submit ours through a company that handles them for us, or you can do them directly. All they do is go to a website and put a request into the database.