Well, according to the manual, parts are containers for events.
Now, when we are talking about audio, when we record something by simply pressing record and then stop after some seconds, what stays on the screen with the waveform inside is an audio event.
But! When we are talking midi, and we record something the same way as before (press rec, play some notes on the midi keyboard, stop), what stays on the screen is a part(!), with events (notes I say) inside. These midi parts I can glue easily from start to end without problems, and if I wish make them in a huuuge continuous block (part to be precise), from project start to project end.
However, when audio is made into parts (for easier handling of a “group” of events, some editing functions are not available.
You can very easily make audio into a part without meaning to do so by glueing. Glueing will heal a cut in an audio event if both edges coincide, but in eeevery other case glueing will just create a part instead, for the two adjacent events it was used on.