The 8-track tape is a magnetic-tape sound recording technology that was popular from the mid-1960s to the early 1980s. To eliminate the nuisance of tape-threading wih reel-to-reel tape recorders, various manufacturers introduced cartridges that held the tape inside a (metal or plastic) housing to eliminate handling. Most were intended only for low-fidelity voice recording eg. in dictation machines. The endless loop tape cartridge was first designed in 1952 by Bernard Cousino around a single reel carrying a continuous loop of standard 1/4-inch, plastic, oxide-coated recording tape running at 3.75 in (9.53 cm) per second.
The popularity of both 4-track and 8-track cartridges grew from the booming car industry. In 1965, the Ford Motor Company introduced factory-installed and dealer-installed 8-track tape players as an option on three of its 1966 models, and RCA Victor introduced 175 Stereo-8 Cartridges from its RCA Victor and RCA Camden labels of recording artists catalogs. By the 1967 model year, all of Ford's vehicles offered this tape player upgrade option. The 8-track format gained steadily in popularity because of its convenience and portability. Home players were introduced in 1966 that allowed consumers to share tapes between their homes and portable systems. By the late 1960s, the 8-track segment was the largest in the consumer electronics market and the popularity of 8-track systems for cars helped generate demand for home units.
The format is obsolete and was relatively unknown outside the US, the UK, Canada, New Zealand, Australia, Germany, Italy, and Japan, although I occasionally saw them at flea markets in the Netherlands.
Read more about 8 track in this Wikipedia article or watch this very cool 8-track documentary, shot in the 1990s: So wrong they're right. Facebook groups: Kate's Track Shack, 8-Track Tapes and Players, Buy and sell group.