What is csound? Its a software based synthesizer that can run on windows, mac, Linux, and there are efforts to port it to open Solaris as well.
You can make any sound you can think of, and then write your own op-code in c if you think of something they haven't.
In fact that's how the system has grown over the years, with user submitted op-codes filling out the available built in system until you have this huge sonic palette.
Unlike vst plug-ins, which while easy to use, the power of the synth is constrained by a graphic interface and a limited set of parameters, with csound you have the full power of a completely modular system.
However its not really free! You have to work to learn how to use it, the learning curve is steep, even with books and tutorials. So you pay in sweat-equity rather than cash.
It is possible to use the csound api to embed csound in a vst, so now you could use csound to develop vst plug-ins for less technically inclined musicians.
But there is a growing community of composers who meet over the csound mailing list to share their music. Since csound compositions are plain text files all that is needed to write music with csound is a text editor.
Now you can use GUI software such as Blue to make it easier to write compositions, but it still takes a kind of technical mindset to work with the software.
Although it lends itself to ambient music, its not limited to that, people have used it to make techno tunes or even renderings of classical scores.
It particularly lends itself to re-working old classical scores since you have total dynamic control of every note, so its easy to reproduce the dynamic nuance of a classical score.
The csounds website has a lot of great resources for those interested in getting started with csound.