This is the "other" consumer audio plug type, the "larger" kind. This kind you see on most "higher-end" equipment; for example, most stereos use it for headphone-out, and some of the slightly more professional equipment uses it for connecting. For example, it's the kind you plug into an electric guitar for audio-out.

Outside of the U.S. you would probably see "6.35 mm" in the name in place of "1/4 inch".

It's important to note that stereo plugs (2-channel) are distinguishable from mono plugs (1-channel) by the number of black stripes on the end of the plug itself. If it has a thick and thin line (thicker towards the end), it's stereo. If it has only one line, it's mono.

Often, when dealing with 1/4-inch stereo plug jacks, you're either using a nice pair of headphones, or you're rigging your stereo system to do something wierd (or maybe that's just me). For instance, I have my computer audio hooked into my (large) stereo system, and it is connected thus:

1/8-inch stereo plug output from sound card >> 1/8-inch stereo plug male via wire to 1/8" female >> 1/8-inch to 1/4-inch stereo plug adapter >> 1/4" to RCA 2-channel adapter >> RCA wire to badass stereo system.

Since there are a lot of adapters in there, it's important for me to not all stereo adapters, noting my little stripes.