Antagonist: Drug occupies receptor, but does not stimulate it. Instead, it prevents NTs from attaching.

Partial Agonist/Antagonist: Drug occupies receptor and only marginally stimulates it. It therefore prevents attachment of NT and overall decreases the action of the NT. As you see below, the drug prevents withdrawal.
Agonist: Drug occupies receptor and stimulates it. The drug therefore mimics the action of NT.