The terms personal communications services and digital cellular systems describe frequencies in the radio spectrum set aside for use by mobile phones and other such devices requiring cellular service.
PCS refers to the cellular services and technologies in the two gigahertz range. In the United States, all of the space between 1,850 megahertz and 1,990 megahertz is owned by the various cellular carriers for their PCS systems.
PCS and DCS are different versions of the same thing: they are both implementations of the Global System for Mobile Communications, or GSM, standard for mobile communication. The main difference is that the two systems operate at different frequencies in the radio spectrum.
DCS refers to certain bandwidth in the radio spectrum reserved for digital cellular communications. In the United States, DCS operates in the 1,800 megahertz range.