Slow-scan television

SSTV (Slow-scan television) is a way of sending a picture on radio waves, it is different from normal television because only one picture is sent very slowly, hence why it's called slow scan, instead of 24 or 30 pictures per second on normal television, SSTV is also not used for entertainment, although radio hobbyists may enjoy collecting pictures that they receive, or seeing if they can receive a picture from a very far away transmitter (DXing).
SSTV is also narrow bandwith, that means the signal is "thinner" it uses much less of the radio spectrum then normal television.
SSTV works by turning a picture into sounds, tones that represent the color and brightness of the lines that make up the picture, these sounds can be transmitted over radio and heard and decoded by other people.
Below is an example of SSTV audio
There are different modes of SSTV, which change the quality of the picture or if it's color, for example, a mode called B/W 8 is very low quality, but it doesn't take that long to transmit, whereas PD 240 is much higher quality, but takes a lot longer to transmit.
SSTV was first used by NASA to beam down pictures of the moon or from the space shuttle, but nowadays it's mainly used by amatuar radio operators and radio pirates, an amatuar radio SSTV picture will often have information on it about the station or person it came from.
The International Space Station still occasionally sends down an SSTV picture or series of pictures.
SSTV is also sometimes used in things that aren't radio, a video game called Portal had SSTV recordings hidden in certain locations, when people decoded them, they revealed more about the storyline in the game.