Quick build system overview

There is a good overview here.

Adding a new builder

There is a good overview here.

Other questions

Can builders share the same host?

Yes. However, build directories can be large and cause the bot to run out of space. Keep this in mind when assigning the same host to multiple builders. If the builders can share the same build directory, specify that using botbuilddir in builders.pyl (if your master has one) or in slave.cfg.