REAPER is a digital audio workstation: a complete multitrack audio and MIDI recording, editing, processing, mixing, and mastering environment. Using your current computer and no other software, you can import any audio and MIDI, synthesize, sample, compose, arrange, edit, mix, and master songs or any other audio projects. If you add a hardware audio interface of your choice (AD/DA: analog-to-digital/digital-to-analog) and a microphone, you have a complete recording studio, suitable for recording anything from a soloist to a band to an orchestra (even if the orchestra is just you).
REAPER converts your computer into the full power of any top-of-the-line recording studio. Minus, of course, a room full of shockingly expensive converters, microphones, amplifiers, and, well, talent. If you are a top-of-the-line recording studio interested in REAPER, this part of the discussion has probably insulted your intelligence.