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".