One of the fastest commercially available connections to the Internet, it is a set of communications standards that enables a single wire (or optical fibre) to carry voice, data, and video. It gives a user up to 56K of databandwidth on a phone line (when also used for voice) or up to 128 Kbps if the line is only used for data.
ISDN is intended to replace eventually the "plain old telephone system" (POTS), and it mostly uses existing Public Switched Telephone Network (PSTN) switches and wiring, upgraded so that the basic "call" is carried on a 64 Kbps channel that's all digital, from end to end. Packet and frame modes are also provided in some places. For more information on ISDN, visit this definition on NetLingo.com.