A cassette single (also known as a "cassingle") is a music single in the form of a Compact Cassette. The format was introduced in the 1980s, when vinyl record album sales were declining in favor of cassette recordings; the cassette single was introduced to replace the 45 record in a similar way.
Originally, most cassette singles were released in a cardboard sleeve that slipped over the outside of the release. This was then shrink wrapped in plastic. Some singles contained one song on each side, much as 45s had done, but others repeated the songs on both sides.
As the cassette maxi-single was released, more intricate packaging was incorporated that looked similar to the packaging of a regular cassette release. These were placed in regular plastic cassette cases with a paper/cardstock insert. Unlike a full-length cassette album, these were generally only one two-sided inlay instead of a fold-out.
Although the cassette had reached a high level of popularity by the late 1980s, due to the ubiquity of mobile devices such as the Sony Walkman, the boombox and car audio cassette players, cassette singles never eclipsed gramophone records to the same extent as cassette albums had done. In the U.S., cassette singles were completely phased out by the mid 2000s. One reason for their lesser popularity was because they appeared to be an inefficient use of the media to consumers — a cassette single took up the same storage space as a full album.
Bow Wow Wow's "C.30 C.60 C.90 Go" was the first cassette single, released in the UK in 1980, and I.R.S. Records released the first cassette single in the U.S. with the Go-Go's "Vacation" in 1982.
Also, cassette maxi-singles were sometimes packaged in longboxes so they could be placed in the same sale bins/racks as CD longboxes which were prevalent (in the U.S. at least) up until 1993.
Cassingles never really gained traction the way CD singles did, and while they have been lampooned as the "redheaded stepchild" of music collectibles, they are a legitimate music collectible. The ones that appear to be the most in demand, and thus garner the highest prices in the collectibles market are imports from the U.K. or Australia, or cassingles that were noncommercially released promos.
My reference collection on this site consists primarily of hard rock, heavy metal, and pop music. Cassingles were also big in the genres of hip-hop, rap, and R&B, and to a lesser extent country music, but since I don't like or collect those genres, you are unlikely to find any level of completeness for those cassingle releases here.