FFmpegSource (usually known as FFMS or FFMS2) is a cross-platform wrapper library around FFmpeg, plus some additional components to deal with file formats FFmpeg's libavformat has (or used to have) problems with. It gives you an easy, convenient way to say "open and decompress this media file for me, I don't care how you do it" and get frame- and sample-accurate access (usually), without having to bother with the sometimes less than straightforward and less than perfectly documented FFmpeg API.
The library is written in C++, but the public API is C-friendly and it's easy to simply include and link directly with a pure C application. The source is available under the MIT license, but the binaries are GPL licensed because they need to be linked against a GPL'd FFmpeg.
Avisynth pluginFor the end user, the most visible use of FFMS is the implementation of an Avisynth source plugin that uses the FFMS library to open media files. This plugin is a part of the FFMS2 project and is available for download here; for documentation see the UserGuide.
These details are provided for information only. No information here is legal advice and should not be used as such.