Here's the basics. FFmpeg was developed for Linux, but there are builds available for Windows. It's in a constant state of development. It's a command-line controlled program (Do you still DOS?) Documentation is poor. Some video transcoding software utilizes FFmpeg. There are also several "FFmpeg GUIs", which come with a particular version of FFmpeg, which is usually outdated. Often the GUI can not interface with a newer version of FFmpeg. FFmpeg has it's own internal encoders which are quite fast. Some builds of FFmpeg also include open-sauce, free, encoders like LameMp3 and XviD, which allow you to use whatever version of the out-sourced encoder was built into the FFmpeg executable/binary/windows program.
FFmpeg includes a myriad of command-line options/switches to control encoding. These are very poorly documented (i.e. range and type of permissible values are not given). If you enter '-h' at a command prompt, you'll get a list of available options/switches which may or may not be up-to-date or applicable to that particular version. Interestingly the documentation at the FFmpeg website doesn't list half as many options as you get from the HELP function.
If you can ever understand all the various options, you may be able to create high-quality encodes. ;-)
If you're a DOS command-line freak, this is your day.
Product reviews by 'fwupow'
I just installed the latest version briefly and it's not ready for prime-time.
I convert *.vob files to DivX and this app won't allow me to select the DivX codec for encoding. I was willing to use Xvid, but even though Xvid is installed properly, it tells me that "Xvid failed to open" etc.
The producer of this app doesn't want to be bothered with writing documentation, but since he is the only one who really knows how it works, he's gonna need to, if he wants anybody to be happy with the program and donate. The app seems to lack very much in the way of error checking and feeding back informative and helpful explanations for various failures/crashes/mishaps.
If you're into the latest H.264 craze, you may have a better experience.

