First, let's start with some definitions.
Agonists are chemical agents which stimulate a certain kind of response.
Usually agonists are compounds which activate a receptor in cell membranes. Antagonists are chemical agents which work against a certain kind of response. Antagonist compounds may block a receptor so that it cannot be activated, or they may activate a different receptor in the same cell and activate a process which works against the process connected to the first kind of receptor. Receptors are protein-based structures embedded in cell membranes which serve as relays for signals. External chemical agents such as hormones or prostaglandins encounter the receptors on the outside of the cell.