T cells are lymphocytes which develop in the thymus.
The youngest T cells have a T cell receptor associated with a protein called CD3. In the thymus, the cells then develop both a CD4 and a CD8 marker. These cells are referred to as double positive. Eventually the cells lose either CD4 or CD8 to become one of the functional subsets. Th and Tc cells both have a TcR, but they perform very different functions in the immune system.