Well it’s the form of magnetic tape audio recording in which the recording medium is held on a reel, rather than being securely contained within a cassette.

In use, the supply reel or feed reel containing the tape is mounted on a spindle; the end of the tape is manually pulled out of the reel, threaded through mechanical guides and a tape head assembly, and attached by friction to the hub of a second, initially empty take-up reel. The arrangement is similar to that used for motion picture film.

The basic idea involves an electromagnet that applies a magnetic flux to the oxide on the tape. The oxide permanently "remembers" the flux it sees. A tape recorder's record head is a very small, circular electromagnet with a small gap in it, like this This electromagnet is tiny -- perhaps the size of a flattened pea. The electromagnet consists of an iron core wrapped with wire, as shown in the figure. During recording, the audio signal is sent through the coil of wire to create a magnetic field in the core. At the gap, magnetic flux forms a fringe pattern to bridge the gap (shown in red), and this flux is what magnetizes the oxide on the tape. During playback, the motion of the tape pulls a varying magnetic field across the gap. This creates a varying magnetic field in the core and therefore a signal in the coil. This signal is amplified to drive the speakers.

The reel-to-reel format was used in the very earliest tape recorders, including the pioneering German Magnetophones of the 1930s. Originally, this format had no name, since all forms of magnetic tape recorders used it. Reel-to-reel tape was also used in early tape drives for data storage on mainframe computers, video tape machines, and later for high quality analog and digital audio recorders in the 1980s and 1990s, before hard disk recording effectively eliminated the need for reel-to-reel technology Even today, many artists of all genres prefer analog tape's "musical", "natural" and especially "warm" sound. Due to harmonic distortion, bass can thicken up, creating the illusion of a fuller sounding mix