The most commonly referred to cluster of differentiation molecules are CD4 and CD8 cells. These are used to indicate the subtype of T cell (helper or cytotoxic), with the ratio of these being a measure of the progression of AIDS.
They get their name from the way the were derived. Monoclonal antibodies are produced against a particular surface molecule on a white cell. If two separate monoclonal antibodies can be produced against a molecule, they are clustered as a new CD.
Because CD molecules, by their very existence as such, are antigenic, antibodies that attach to them can be manufactured. These can be used in flow cytometry to count the number of cells of a particular subtype.
Although the CD molecules are useful as a way of differentiating cells, they exist to serve a purpose for the cell carrying them.