OpenAL aims to provide a standardized API for something called '3D' sound, just like OpenGL does for '3D' video rendering.
The fun part of OpenAL is that it does all the DSP for you, so you don't need to know a lot about audio. Since OpenAL comes from Creative Labs, it supports hardware rendering of audio with most if not all SoundBlaster audio controllers. They come with chips from EMU-systems and are specifically designed to mix down multichannel audio with DSP effects like reverb and chorus, which could consume a lot CPU speed.
The downside of OpenAL is that (at this time of writing) it is focus on games only. It doesn't do MIDI at all, it doesn't record anything, it doesn't work like a synthesizer. The only thing you can do with OpenAL is play sounds in a simulated 3D environment.
It probably doesn't work on the web.
To use OpenAL you need to read the documentation about OpenAL.
Note concerning audio users: there's no such thing as '3D' audio with speakers. Technically, when people have only two ears, they really can't distinguish sound from above or below, even in front or from behind. To achieve this people merely hear volume, the brightness of sound and the phase difference. One should be able to hear more '3D' sound with only two quality-class speakers than with a consumer class 5.1-set.