The Buildmaster runs a collection of Builders, each of which handles a single type of build (e.g. full versus quick), on one or more build slaves. Builders serve as a kind of queue for a particular type of build. Each Builder gets a separate column in the waterfall display. In general, each Builder runs independently (although various kinds of interlocks can cause one Builder to have an effect on another).
Each builder is a long-lived object which controls a sequence of Builds. Each Builder is created when the config file is first parsed, and lives forever (or rather until it is removed from the config file). It mediates the connections to the buildslaves that do all the work, and is responsible for creating the Build objects - see Build.
Each builder gets a unique name, and the path name of a directory where it gets to do all its work (there is a buildmaster-side directory for keeping status information, as well as a buildslave-side directory where the actual checkout/compile/test commands are executed).
A builder also has a BuildFactory, which is responsible for creating new Build instances: because the Build instance is what actually performs each build, choosing the BuildFactory is the way to specify what happens each time a build is done (see Build).
Each builder is associated with one of more BuildSlaves. A builder which is used to perform Mac OS X builds (as opposed to Linux or Solaris builds) should naturally be associated with a Mac buildslave.
If multiple buildslaves are available for any given builder, you will have some measure of redundancy: in case one slave goes offline, the others can still keep the Builder working. In addition, multiple buildslaves will allow multiple simultaneous builds for the same Builder, which might be useful if you have a lot of forced or “try” builds taking place.
If you use this feature, it is important to make sure that the buildslaves are all, in fact, capable of running the given build. The slave hosts should be configured similarly, otherwise you will spend a lot of time trying (unsuccessfully) to reproduce a failure that only occurs on some of the buildslaves and not the others. Different platforms, operating systems, versions of major programs or libraries, all these things mean you should use separate Builders.