OFDM (orthogonal frequency division multiplexing) is a technique for increasing the amount of information that can be carried over a wireless network.
In frequency-division multiplexing, multiple signals, or carriers, are sent simultaneously over different frequencies between two points. However, FDM has an inherent problem: Wireless signals can travel multiple paths from transmitter to receiver (by bouncing off buildings, mountains and even passing airplanes); receivers can have trouble sorting all the resulting data out.
Orthogonal FDM deals with this multipath problem by splitting carriers into smaller subcarriers, and then broadcasting those simultaneously. This reduces multipath distortion and reduces RF interference (a mathematical formula is used to ensure the subcarriers' specific frequencies are "orthogonal," or non-interfering, to each other), allowing for greater throughput.
OFDM is at the heart of 802.11a wireless-LAN technology, which can offer throughput of up to 54M bit/sec (the 802.11a specification calls for up to 52 subcarriers). It is also used in ADSL over copper telephone wires.