Ligands are small molecules that transmit signals in between or within cells. They exert their effects by binding to cellular proteins called receptors. The ligand is like a baton, and the receptor is like the next runner in line. After binding to the ligand, the receptor can then send additional signals to other parts of the cell.
There are two main types of ligands: ligands that bind to receptors inside the cell, called intracellular ligands, and ligands that bind to receptors outside the cell, called extracellular ligands.