This is probably the type of audio plug that people in general will come into contact with most frequently-- you've almost certainly got at least one device somewhere in your house that uses one of these. It's the "small" kind, the kind that you see on almost all headphones, the kind you plug in to all walkmen, and the kind you plug in to almost all computers for audio-out and (usually) audio-in.

You sometimes see this plug type going by the name "1/8-inch stereo miniplug" or sometimes even "1/8-inch mini-stereo plug". Outside of the U.S. you would probably see "3.5 mm" in the name in place of "1/8 inch".