Computer-Mediated Communication (CMC) is any form of communication between two or more individual people who interact and/or influence each other via separate computers. Notice that this does not include the methods by which two computers communicate, but rather how people communicate using computers.
The consequences of switching communication to a more computer mediated form include altered: impression formation, deception and lying behavior, group dynamics, disinhibition, and especially relationship formation.
CMC is examined using three main aspects of any form of communication, that are altered by the medium used; these aspects are synchronicity, persistence, and anonymity. Each of the aspects vary widely for different forms of communication. Instant messaging is highly synchronous, while not persistent. Message boards are low in sychronicity, but high in persistence. Anonymity depends more on the context and particular program/web page being used. It is important to remember the psychological and social implications of these factors, instead of just focusing on the technical limitations.
Currently, one of the main explanations for a lot of effects in CMC is the Hyper-Personal Model, developed by Joseph Walther of Cornell University.