What's the difference between a ligand and a receptor?

A ligand (in the context of biochemistry, not chemistry) is a general term for any molecule that binds to another molecule. Examples would include a hormone binding to a receptor, oxygen binding to haemoglobin, or antibodies binding to antigens.

A receptor, on the other hand, is a protein to which a signalling molecule (such as a neurotransmitter, drug, or hormone) binds specifically and stimulates a particular response by a cell.

So, in other words, the generic term for the things that bind to receptors is... ligands.
Conversely, although ligands can bind to receptors, they can also bind lots of other molecules.

Hope that helps!

Source: Henderson's Dictionary of Biology (13th Edn.), Ed. Lawrence