A Composition is Mass' idea of a project. It is a set of rules and constraints combined with semi-free elements which together create generative MIDI output.
For more info, see Compositions
Parts are the primary actors within a Composition. They are self-contained event generators that can interact with each other and the Composition itself. All events generated within a Composition come from Parts.
For more info, see Parts.
Events create the MIDI data that is output to the system. Each Event in a Composition is a specific function designed to output a specific MIDI message, or to influence the Composition in some way. Events have controls that affect their function, and these controls change based on the Event's type.
For more info, see Events.