Although disco music is generally acknowledged as an important turning point in pop, both in that it was the beginning of the truly studio albums, as well as the origin of house, techno, and just about any other style that uses a beatbox, disco is much-maligned today. Unlike its successors, disco was usually recorded using traditional acoustic instruments. Music historians also will tell you that disco never went anywhere, that it just underwent metamorphosis.
Nevertheless, when the word disco is uttered, most people associate it with the dance music of the late seventies (or should I now specify 1970's?). In addition to the characteristics mentioned above by Cyt, most music in this style was in a minor key, used real string instruments (at least in the old days). The bass line is probably the most crucial part of this sound, since this is what identifies the sound to many people: usually fast, busy, and lots of walking bass and/or wide leaps.