An agonist will active receptors, causing a physiological response. (Pretty much, it acts like a neurotransmitter). On the other hand, an antagonist will BLOCK a receptor, preventing a neurotransmitter from binding on that receptor, therefore blocking a physiological response.
Agonist --> acts like a neurotransmitter and elicits a responseAntagonist --> BLOCKS a neurotransmitter by binding on to the receptor, therefore blocking a response.