The Compact Cassette, also called audio cassette, cassette tape, cassette, or simply tape, is a magnetic tape sound recording format.

Source: http://en.wikipedia.org/wiki/Compact_Cassette.

The Compact Disc (also known as a CD) is an optical disc used to store digital data. It was originally developed to store and play back sound recordings only.

Source: http://en.wikipedia.org/wiki/CD.

Digital download refers to a non-physical audio product usually in .mp3 or .wav formats.

For further information: http://en.wikipedia.org/wiki/Digital_download.

DVD is an optical disc storage format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions. DVDs are used in DVD-Video consumer digital video format and in DVD-Audio consumer digital audio format, as well as for authoring AVCHD discs. DVDs containing other types of information may be referred to as DVD data discs.

Source: http://en.wikipedia.org/wiki/DVD.

The 12-inch single (often simply called 12") is a type of gramophone record that has wider groove spacing compared to other types of records. This allows for louder levels to be cut on the disc by the cutting engineer, which in turn gives a wider dynamic range, and thus better sound quality. This record type is commonly used in disco and dance music genres, where DJs use them to play in discos or clubs. They are played at either 33 1/3 or 45 rpm.

Source: http://en.wikipedia.org/wiki/12%22_vinyl.