Broadcast television requires huge 6 MHz-wide (6-million cycle-per second) channels, because it transmits roughly thirty pictures per second. SSTV is less greedy, usually only taking up 3kHz (3000 cycle per second channels). SSTV can transmit data much more slowly because it sends still pictures, usually one every ten to thirty seconds. The technical term for this is to say that SSTV is narrowband.
SSTV originally required quite a bit of specialized equipment. Usually there was a scanner or camera, a modem to create and receive the characteristic audio howl, and a cathode ray tube from a surplus radar set. The special cathode ray tube would have "long persistence" phosphors that would keep a picture visible for a minute or so.
The modem would generate audio from pictures, and pictures from audio. The audio would be attached to a radio receiver and transmitter.
The modern system uses a personal computer and special software in place of much of the custom equipment. The audio system of a personal computer, with custom software, acts as a modem. The computer screen provides the output. A small digital camera, or digital photos provide the input.
See also radiofax, and amateur television.