New to CA and confused over overtime laws

Curious about the overtime issue. Currently work per diem and my agency pays me 4 hrs overtime on every 12 hr shift (overtime kicks in after 8 hrs) at the local hospital. I thought the agency did this because required by law. When i talk to permanent nurses at the hospital they indicate their not paid overtime on 12 hr shifts only after 40 hrs in a week. What is the scoop about overtime in California? Do employers have to pay overtime after 8 hrs in a shift? Thanks for clarification by anybody in the know.

Ok, this is how it works. In California if you work over 8 hrs in any pay period they are required by law to pay you over time. HOWEVER...there are loop holes. At my job they only pay over time if you go over 40 hrs in a week. The employees "agreed" to this and signed a form stating that this was ok and that they "agreed" to this set up. I say "agreed" in quotations because this happened before I started working their and the people that signed this said they they were tricked. They said management made it sound like it was a benifit for them(the employee), "This way if you want to work four ten hour shifts you can have three day weekends!" Well to say the least that hardly ever happens, people are scheduled monday through friday, and if they work an hour over here and there they do not get paid over time.......