Affinity chromatography
Affinity chromatography works because most molecules from inside living things stick to some molecules but not others, the way magnets stick to opposite magnets but not to wood or plastic.  An antibody will stick to its antigen but not to a different antibody's antigen.  A protein will stick to its own receptor or inhibitor but not to a different protein's receptor or inhibitor.  A DNA-splicing protein will stick to its own DNA sequence but not to a different DNA sequence.  In affinity chromatography, the thing that the target molecule sticks to is called the ligand.