Teapot Rebellion
A balun or unun stands for balanced to unbalanced, or unbalanced to unbalanced coupler. Typically a balun is used when connecting an unbalanced coaxial cable to a balanced antenna such as a dipole, which has two equal quarter wavelengths of wire at the desired frequency. These devices are usually made up of a ferrite core with two sets of windings, one to the transceiver and one to the antenna. The balun helps prevent the outer sheath of the coaxial cable from picking up the transmitted signals and feeding them back into the transmitter, depending on the length of the coaxial cable being used. By varying the ratio of turns, they can also be used for impedence matching. A single winding may also be used and connected as an auto transformer.
So essentially a balun or unun is just a transformer used to couple or connect the transceiver to the antenna cable. The ARRL and RSGB internet sites contain detailed information on use and construction of these devices.