Antenna: An antenna is a device for converting electromagnetic radiation in space into electrical currents in conductors or vice-versa, depending on whether it is being used for receiving or for transmitting
http://www.cv.nrao.edu/course/astr534/AntennaTheory.html
http://www.cv.nrao.edu/course/astr534/AntennaTheory.html
Antenna Fundamentals
Gain: Gain is the measure of an antenna to direct a specific radio frequency energy in a specific direction, measured in Decibels (dB). Gain is a measurement of how much the energy has increased by using amplification.