Composite video is the format of an analog television signal before it is modulated onto an RF carrier. It is usually in a standard format such as NTSC, PAL or SECAM. It is a composite of three source signals called Y, U and V (together referred to as YUV). Y represents the brightness or luminance of the picture and includes synchronizing pulses, so that by itself it could be displayed as a monochrome picture. U and V between them carry the colour information. They are first mixed with two orthogonal phases of a colour carrier signal to form a signal called the chrominance. Y and UV are then added together. Since Y is a baseband signal and UV has been mixed with a carrier, this addition is equivalent to frequency-division multiplexing.
Composite video can easily be directed to any broadcast channel simply by mixing it with the proper RF carrier frequency. Most home video equipment records a signal in composite format: VCRs and laserdiscs both work this way, and then give the user the option of outputting the raw signal, or mixing it with RF to appear on a selected TV channel. In the United States, the composite video signal is typically connected using an RCA jack, normally yellow (with red and white for left and right sound). In Europe, a coax connector or SCART connector is used.
Some devices that connect to a TV, such as videogame consoles (and the ubiquitous home computers of the 1980s), naturally output a composite signal. This may then be converted to RF with an external box known as an RF modulator that generates the proper carrier (often for channel 3 or 4 in North America). The RF modulator is preferably left outside the console so the RF doesn't interfere with the components inside the machine. VCRs and similar devices already have to deal with RF signals in their tuners, so the modulator is located inside the box. Also, most home computers usually employed an internal RF modulator.
The process of mixing the original video signal with RF, and then removing the RF again in the TV, introduces several losses into the signal. RF is also "noisy" because of all of the video and radio signals already being broadcast, so this conversion also typically adds noise or interference to the signal as well. For these reasons, it's typically best to use composite connections over RF connections if possible. Almost all modern video equipment has composite connectors, so this typically isn't a problem.
However, just as the mixing and removal of RF loses quality, the mixing of the various signals into the original composite signal does the same. This has led to a proliferation of systems such as S-Video and component video to separate out one or more of the mixed signals.