I've been looking around for a better job but have worked a lot of sort odd (temporary) jobs. I have been job hopping for a little over a year. This looks bad to prospective employers. Does HR simply call previous employers and rely strictly on what the former bass says or is there a national database that lists EVERY job has ahd in there life?
Can they find out how long you've lived at an address?

Depends on who you are applying with. If you are applying with the federal government and the position requires clearance, you are going to get a thorough background check that will include credit, previous jobs, criminal and where you have lived.

Small businesses like the one I work for will do a former employer check. The big call center I worked for years ago did a credit check only.

The problem with listing your jobs is that you look like you aren't reliable if you list several jobs within one year period. However, it looks just as bad if you omit or try to make one look like it lasted longer than it did and that reference is the one they decide to check up on.

As someone that has done hiring I can tell you that I give more consideration to someone in their early 20s that has job hopped than I do someone who is in their early 30s. Sometimes it takes a few crappy jobs to figure out what you DON'T want to do in life.

They just call the bosses you are putting down,

If you are going to work for some licensed jobs, police, corrections, National secuirty jobs, airport, insurnce agents, real estate, they will have a much more detailed reseach