What do you call the catchy bit of a song that everyone remembers and that repeats?

What do we call the way music is organized into sections?

When naming the sections of a musical form, do you label each part with numbers or letters?

This is a special kind of form that uses the pattern ABACA for it's sections.

Which one of the answers is NOT a form?

What Form is represented by the pattern of shapes you see here?

This is the part of a song that does NOT repeat. It is not the most memorable part.