The 8-track cartridge was a magnetic tape format, popular for music from the mid-1960s to the early 1980s. It was created by Bill Lear (the Lear Jet guy) in 1964. 8-tracks replaced a similar 4-track tape system, called Stereo-Pak, invented by Earl "Madman" Muntz (known for crazy car commercials and cheap TVs).
The tape within the 8-track cartridge was arranged in an "endless loop" and coated with a slippery material to minimize the friction where the tape rubbed against itself. The coating sometimes also caused the tape to slip, leading to poor speed control, which hurt the sound quality and made the cartridges unpopular with audiophiles.
The design allowed simple, cheap, and mobile players; but unlike a two-reel system, it didn't permit tape movement in both directions. Some players offered a limited fast-forward function, but rewinding was impossible.
The 8-track cartridge was briefly used for 4-channel "quadraphonic" recordings, but was made obsolete by the Compact Cassette, invented at Philips in 1963. The cassette was originally intended to be a monophonic dictation device with no consideration for high fidelity.
The stereo "Music" audio cassette (or Musicassette) was introduced in 1966, and became a practical high fidelity format with the addition of Dolby noise reduction in 1971. (Your editor attended the press conference where Dolby Labs first demonstrated cassettes with their noise reduction.)
Cassettes not only sounded better than 8-track tapes, they were much smaller, stored more music without breaks, and could be recorded in home recorders, which were uncommon in the 8-track format. During the transitional period in the 1980s, there were adapters that fit into automotive 8-track players to allow playback of cassettes without a big investment.
8-track players became less common in homes and automobiles as the 1970s went on. By the time the Compact Disc arrived in 1982, the 8-track had nearly disappeared. 8-tracks were phased out of retail stores by 1983.
There is a debate about the last commercially released 8-track by a major label, but many agree it was Fleetwood Mac's Greatest Hits in November 1988. Some 8-track titles were still available through record clubs until 1989. Many of these late-period releases are highly collectible due to the low numbers that were produced. Among the most rare is Stevie Ray Vaughan's Texas Flood. The record-club-only 8-track cartridge that seems to sell for the highest amount is The Police's The Singles, which has sold for over $200 for a single copy. Another highly sought-after title among collectors has been The Sex Pistols' Never Mind The Bollocks Here's The Sex Pistols, which has sold for over $100 for an open copy in average condition.
There was also a rare record club only 8-track box set of Bruce Springsteen and the E Street Band's Live/1975-85, which is probably the only boxed set ever released on vinyl, cassette, compact disc and 8-track tape. There are reports of bootleg 8-track tapes being made in Mexico as late as 1995, and some independent artists released 8-track tapes as late as 2006. (info from Wikipedia & other sources)