Being able to decode most video/audio files found on the net from common WMA/WMV/MPG/OGG/AVI to the rarest formats AU/MOD/MonkeyAudio, it is the equivalent of MS DirectShow on steroids.
Highly modular and with a clean code base, it runs on most Unixies (Linux, MacOSX, Solaris, ...) and also under Windows.
Designed as a set of plug-ins, you can encode/decode/stream whatever you want.
It is even business friendly as you can find commercial support easily via Fluendo, Collabora Multimedia and certainly many more OSS companies.
As its license is LGPL and it is set of modules, they could build their own proprietary modules and use them. (to use a hardware en/decoder for example)
Feel free to jump into #gstreamer on Freenode if you have questions. They're always responsive and friendly.