PulseAudio is a sound system for POSIX OSes, meaning that it is a proxy for your sound applications. It allows you to do advanced operations on your sound data as it passes between your application and your hardware. Things like transferring the audio to a different machine, changing the sample format or channel count and mixing several sounds into one are easily achieved using a sound server.
PulseAudio is designed for Linux systems. It has also been ported to Solaris, FreeBSD, NetBSD, MacOS X, Windows 2000 and Windows XP, but we don't currently have maintainers using non-Linux systems, so if you're having issues on those systems, you get to fix them yourself (patches are welcome). Windows binaries are not provided by the upstream PulseAudio project. PulseAudio is an integral part of all relevant modern Linux distributions and used in various mobile devices by multiple vendors.