A receptor is a protein, usually on or in a cell, that a molecule can bind to and, through that binding, trigger a biological response. A ligand is any molecule that binds a receptor — the general term for “the thing that attaches.”
An agonist is a ligand that binds a receptor and activates it, producing the biological response the receptor is capable of triggering. An antagonist binds the same receptor but does not activate it — often blocking the receptor from responding to its normal agonist instead.
Receptor selectivity describes how narrowly a molecule targets one receptor versus binding several related ones, and binding affinity describes how strongly and readily a ligand binds its target. A “pathway” refers to the downstream chain of biological events a receptor sets off once activated.