How does a plasmid work?

[WizardofScience]

I'm having difficulty understanding how exactly a plasmid does what it does. I know that it's generally located in bacteria, and it's non-essential to them. I don't understand how they replicate though. Does it work the same way as a bacterium's chromosomal DNA?

Thanks for any help!

[JackBean]

Basically yes, but the replication start is not that much strict (thus you can have up to several hundreds of copies in one cell).

[WizardofScience]

Can you (or anybody else) go into a little more detail about how the replication works? Or link me to a website? Is it still the same with helicase unwinding the DNA and primase coming in and etc. like in regular DNA replication?