The purpose of this document is to document the way some particular learning algorithms (like Deep Belief Networks) are implemented using PLearn's base classes. It is not to detail how those base classes are working.
You should read PLearn programmer's guide first (or at least have it reachable), you will need it for information about PLearn's generic classes, especially Object and PLearner, but also Var and OnlineLearningModule, and for the general coding philosophy.