What is Citizen Band (CB)?

Citizens' Band radio (often shortened to CB radio) is, in many countries, a system of short-distance radio communications between individuals on a selection of 40 channels within the 27-MHz (11 m) band. The CB radio service is distinct from FRS, GMRS, MURS, or amateur ("ham") radio. In many countries, CB does not require a license and, unlike amateur radio, it may be used for business as well as personal communications. Like many other two-way radio

services, Citizens' Band channels are shared by many users. Only one station may transmit at a time. Other stations must listen and wait for the shared channel to be available.

Over time, several countries have created similar radio services, with varying requirements for licensing and differing technical standards. While they may be known by other names, such as General Radio Service in Canada, they often use similar frequencies (26 to 28 MHz), and have similar uses, and similar difficulties with antennas and propagation. Licenses may or may not be required, but eligibility is generally simple.

Some countries have personal radio services in the UHF band, such as the European PMR446 and the Australian UHF CB. Like the American FRS and GMRS services, these are more properly covered in their own articles, as much of this article is specific to the antenna and propagation of the upper HF and lower VHF bands.

For more detail you can click link below:-

http://en.wikipedia.org/wiki/Citizens%27_band_radio