Commonly used terminology for a series of influential drum machines made by Roland in the 1980s; the terminology comes from the way Roland numbered these models -- 606, 707, etc. The first of these was the TR-808, introduced in 1980. This was Roland's second drum machine with user-programmable patterns, after the CR-78; it marked a transition from the '60s-style drum machine which was intended mainly to be an auxiliary device for an organ or piano, to a stand-alone unit packaged in the now-common tabletop style. This was shortly followed by the TR-606 Drumatrix, which was intended to be paired with the TB-303 bass synthesizer; the pair were intended to be used as accompaniment for e.g. a guitarist to practice without a bassist and drummer. The 808 was superseded in 1983 by the TR-909, with improved programming capability and more flexibility in its drum sounds.
Unfortunately for Roland at the time, these models still relied on analog synthesis techniques, and in the market it faced off against the Linn LM-1; the Linn was introduced the same year but it used sample playback methods. Analog synthesis has great difficulty creating realistic drum sounds, and given that the basic technology had been in use in drum machines since the '60s, the sounds were regarded as "cheesy" and reminiscent of cheap home organs. The Linn, by contrast, had samples of real drum sounds, and it vastly outsold the Roland units at the time. (And it didn't help any that the 606 in particular was flimsily built and felt cheap.) Roland finally introduced a digital machine, the TR-707, in 1984. This was an innovative unit that graphically displayed a drum grid on an LCD for drum pattern editing, but it came to the market far too late.
Flash forward to 1990: The Linn drum sounds have been so over-used over the past decade that everyone is tired of them. The long-ignored Roland machines, particularly the 808 and 606, can be found cheaply in pawn shops. House music is starting to appear, and the musicians and DJs of this genre have discovered that, by tweaking trim pots inside these units, they can create the earth-shaking subsonic kick drum sound that is now a feature of the genre. A cottage industry begins to grow doing modifications of 606s, 808s, and 909s to provide different sounds, and the units that sounded dated in 1980 are now a fresh alternative to the overused digital drum samples. The Roland analog drum units, as well as the TB-303, have now become staples of nearly all forms of electronica. Roland has made repeated attempts to capitalize on its late-but-good fortune by using the X0X model number pattern for more recent drum machine models. Third parties have also attempted to get in on the act, with emulations (which usually include both drums and TB-303-like bass) such as the X0XBox and the Future Retro 777.