Multitrack Recorder, earlier Versions realized in analog Hardware, as 4-Track Tape with all resulting effects in signal quality loss, more modern Versions as single-part digital Hardware.
With faster CPU design and wider busses, the average Computer had enough Power to process Audio Signals. So the usual Multitrack Recorders these days are mostly Software that enables the User to record a theoretical infinite Number of Tracks, apply different Effects at will, delete, cut and mix all selected channels together freely. While older Software only worked for Audio Signals, the more modern Multitrack Recorder are able to work on Video Signals, too.
The future of those Programs seems to be full Network Transparence (so you don't have to meet to record and mix your stuff together) and additional functions which are currently represented in standalone programs.
Current Multitrack Recorder Software is available in different qualities for different OS. While professional Studios mostly work with Apple/Macintosh and ProTools (tm), this is far too expensive for the standard User. The cheapest solution seems to be a Linux box with ecasound, snd or similar programs.
Even then, the Hardware is still an interesting issue, not only the Motherboard, RAM, CPU and Harddisk, but also the Soundcard, which can vary from a cheap Soundblaster (tm) clone up to any high-end Hardware the User can efford. Plus, if you use Acoustic Instruments, like an acoustic Guitar or even a grand Piano, you shouldn't be stingy when to buy a good Microphone. Many a cheaper Soundcard requires a pre-amp to connect a good microphone to it.
A nice startup to read is Home Recording on a Budget, even if it could use some updates.