Portastudio

The TASCAM Portastudio was the first four-track recorder based on a standard compact audio cassette tape. The term "portastudio" is exclusive to TASCAM, though it is generally used to describe all self-contained cassette-based multitrack recorders dedicated to music production. The Portastudio, and particularly its first iteration, the Teac 144, is credited with launching the home-recording wave, which allowed musicians to cheaply record and produce music at home, and is cited as one of the most significant innovations in music production technology.
The Teac 144 Portastudio made its debut in 1979, at the annual meeting of the Audio Engineering Society. It was followed by several other models by TASCAM, and eventually by several other manufacturers.
For the first time it enabled musicians to affordably record several instrumental and vocal parts on different tracks of the built-in four-track cassette recorder individually and later blend all the parts together, while transferring them to another standard, two-channel stereo tape deck (remix and mixdown) to form a stereo recording.
The Tascam Portastudio 244, introduced in 1982, improved upon the previous design with overall better sound quality and more features, including: dbx noise reduction, dual/concentric sweepable EQ's, and the ability to record on up to 4 tracks simultaneously. 
In general, these machines were typically used by musicians to record demos, although they are still used today in lo-fi recording. The analog portastudios by TASCAM (a division of TEAC) and similar units by Fostex, Akai, Yamaha, Sansui, Marantz, Vestax, Vesta Fire, TOA, Audio-Technica, Peavey, and others generally recorded on high-bias cassette tapes. Most of the machines were four-track, but there were also six-track and eight-track units. Some newer digital models record to a hard disk, allowing for digital effects and up to 32 tracks of audio.
Function.
The Portastudio supported the bouncing of content between tracks, such as creating a mix of three tracks and recording the sum onto the fourth track. By carefully balancing the volume and equalization, this could be done multiple times to create very lush tracks, reminiscent of the complex four-track production of late-sixties Beatles compositions. A limitation was the introduction of tape hiss, which was a particular issue with early models. This was eliminated with more recent digital models.
Unlike standard audio cassette machines that recorded a pair of stereo channels per side, the cassette-based Portastudios recorded four channels in one direction. If you played back a standard two-sided stereo cassette, the second two channels would play in reverse. One interesting (but unpublished) feature was that you could record a track in reverse by flipping the cassette, then flip it back and record additional material. Thanks to the speed control, it was also possible to create a classic flanging effect by recording one track, then recording a second copy of the same track while varying the pitch control, creating a sweeping sound at points where the two tracks converged.
An additional feature was support for external signal processing during recording and on final output.