Cells are in constant communication with one another.  Cells communicate by sending and receiving signaling molecules called ligands.  If ligands are the way that a cell speaks then cell surface receptors are the way that cells listen.  If a ligand connects to the cells receptor then a message is conveyed to the cell.  Different forms of communication depend on the ligand because a receptor can only take its particular form of ligand.

The picture below represents a cell.  Such a cell may have many types of receptors on its surface.  In order to see these receptors we would have to get closer to the cells surface, until we can see the actual molecules that make up the cell.
