What is the difference between occurrence copy and definition copy?

An occurrence copy is only the copy of a graphical representation of an object. Thus, when you create an occurrence copy, a graphic is created that references the copied object:

Occurrence copy

When you create a definition copy, an entirely new object is created whose attributes are identical with those of the copied object:

Definition copy

If attributes of an object are changed, this change also applies to the corresponding attributes in all occurrences of the object. This becomes clear when changing the Name attribute:

Occurrence copy and definition copy

See also

Paste copied object as an occurrence copy

Paste copied object as a definition copy