A ligand is a substance (usually a small molecule), that forms a complex with a biomolecule to serve a biological purpose. In a narrower sense, it is a signal triggering molecule, binding to a site on a target protein or an aptamer. The binding occurs by intermolecular forces, such as ionic bonds, hydrogen bonds and van der Waals forces.
Ligand binding to an aptamer alters the aptamer's chemical conformation (three dimensional shape). The conformational state of a receptor determines its functional state.
The tendency or strength of binding is called affinity.