They are ligands that can act on different parts of receptors and regulate activity in a positive, negative, or neutral direction with varying degrees of efficacy.
Typically, a chemical acts in an agonist fashion whenever it instigates or else facilitates a particular reaction by binding to a particular receptor.
In contract, a chemical acts as an antagonist whenever binding to a particular receptor blocks or inhibits a particular response.
An example is found in medications used to treat opioid addiction, with methadone, buprenorphine, naloxone, and naltrexone all in separate categories or in more than one simultaneously.
Another example is insulin, which activates cell receptors to instigate blood glucose uptake.
Generally, antagonists can act one of two ways: 1) they can either block the receptors directly, preventing the usual ligand from binding, such as in the case of atropine when it blocks specific acetylcholine receptors to provide important medical benefits.