Best way to explain would be to take two examples from two categories.
Composite from Structural patterns defines a tree like structure, so focuses on the relationship. One to many and has a type of relationships so that whole and part can be treated alike.
Observer pattern on the other hand from Behavioral design patterns focus on communication. How do we let the interested parties know any changes to the object. Sort of publisher to subscriber. Does not define a strict structure, but forces on implementing methods, i.e. channels of communications.
Hope it's useful.
