An antenna is a device used to transmit and receive electromagnetic waves such as radio waves and microwaves. Antennas provide the transition between a guided wave (flowing in a wire) and a free space wave (flowing in air or vacuum). An antenna can take high frequency pulses from an electrical signal generator, focus them, and launch them into space, like the antenna at a radio station. Conversely, it can pick up waves from space, focus them, and send them to a receiver, like the antenna on your car radio. You can think of an antenna as a soap bubble pipe: pulses (soap film) travel down the transmission line (pipe stem), reach the bowl (antenna), and are electrically shaped and pushed out into free space. The horn antennas used for microwave communication are designed to let the radiation spread out gradually rather than undergo an abrupt transition from the waveguide into free space. This is known as impedance matching, and contributes to the propagation of the radiation in the same way as cupping your hands around your mouth when shouting makes your voice travel further.
Basically there are two types of antennas: those that rotate and those that are stationary. Rotating antennas usually operate as search and detection systems. They are typically found on ships, airports, or weather stations. Often, an antenna will include a reflecting element to focus the radio waves, commonly parabolic or shaped something like an orange slice.
The stationary antenna type is generally found at radio or microwave transmitting sites. This antenna configuration can be a long wire between pylons, a single pylon with a long rod at the top, or include a number of unevenly spaced rods like an outdoor television antenna. The satellite dish, an antenna with a parabolic reflector, is another common type of stationary configuration.
See also Radar.