CDPD (Cellular Digital Packet Data) is a protocol for wireless data connectivity. It uses the same frequency range as analog cellular (AMPS) and is limited to 19.2Kbps. It will probably start to disappear around 2002.
CDPD basically plays the same role as Ethernet - it provides a hardware layer over which TCP/IP packets can be sent. CDPD devices can appear to the system as modems (emulating SLIP) or as network cards.