A reel to reel tape recorder is a piece of audio equipment that records and plays back sound via a spool of magnetic tape. The full spool is loaded onto the machine (usually on the left side) and the tape is "threaded" past a recording and playback head assembly (which records or reads the magnetic patterns on the tape) and capstan/pinch roller assembly (a steel shaft that ensures constant tape motion) to an empty "take up" reel which collects the tape and winds it. A reel to reel tape recorder is normally used, so that the recorded sound on the tape provides repeatable and reliable results. The tapes are usually edited beforehand to ensure that only the required information is analyzed.

In a cassette recorder, both tape reels stop when the tape reaches the end of a reel, because the tape is positively secured to both reels, and power is shut off instantly. In a conventional reel to reel recorder a slight time delay occurs at the end of the winding operation as the supply reel slows and stops. The name arose only with the need to distinguish it from the several kinds of tape cartridges or cassette s which were introduced in the early 1960s . Thus, the term "reel-to-reel" is an example of a retronym . Of course, 3M and Wollensak got into cassettes and 8-tracks as well. They were a major player in the audio equipment market for many years and Wollensak tape recorders were sometimes considered higher end audio equipment.

Inexpensive reel-to-reel tape recorders were widely used for voice recording in the home and in schools before the Philips "compact cassette ", introduced in 1963, took over. Cassettes quickly displaced reel-to-reel recorders for consumer use.