First you need to get used with some Bamboo concepts (also see the official Atlassian Bamboo documentatiom
A build plan is the process that generates a Bamboo artifact. An artifact is something that can be deployed later, most of the time an executable or a jar file when you are talking about software that compiles, but for our sites this will simply be a compressed tarball file called site.tar.gz that contains the source code.
Build plans are composed of three pieces: Stages, Jobs and Tasks. If you look at the graph below it should be clear how those three fit inside each other: