From Mad Gender Science!

A receptor is a protein that is located on the surface of cells or inside of cells and that modulates cell function. Ligands are compounds that bind to receptors and influence their function. They include agonists, which activate receptors and induce modulation of cell function by receptors, and antagonists, which prevent activation of receptors and stop them from modulating cell function. Examples of receptors include the androgen receptor, the estrogen receptor, and the progesterone receptor, which are nuclear receptors (i.e., are located inside of cells and, upon activation, move to the cell nucleus and bind to DNA, thereby influencing the expression of genes in the cell).