What's all this stuff?

O.k. I'm reading my Bass Player magazine looking at all the new nice basses and they start talking about pickups. Then they start talking about pre-amps. Then they are talking about jazz pickups and standard pickups and single coil and double coil. So what does a preamp do? Where is it located? What's the difference in a jazz and a standard pickup? And what's the difference between the single and double coil pickup? Thank you for your patience.

 

Thanks again. My profile's been filled out. I was wondering what the pre-amp does. My pickups are active.