FFmpeg and Libav are libraries for multimedia decoding (and more). Both libraries expose almost the same API and features.

Note that both libraries actually come as a set of libraries, and both projects use the same library names. The libraries are named libavcodec, libavformat, libavutil, and more. There is a FFmpeg libavcodec and a Libav libavcodec.

Many things in mpv, including video decoding, are done via FFmpeg or Libav. It's a major mpv dependency, and differences between FFmpeg and Libav can have a major impact on its behavior: the number of files it can decode, whether it decodes correctly, what video and audio filters are provided, network behavior, and more.

In 2011, parts of the FFmpeg developers were unhappy about the FFmpeg leadership, and decided to take over. This didn't quite work out. Apparently Fabrice Bellard, original FFmpeg developer and owner of the ffmpeg.org domain name, decided not to hand over the domain name to the new maintainers. So they followed Plan B, and forked FFmpeg, resulting in Libav. Since then, Libav did its own development, and completely ignored whatever FFmpeg did. FFmpeg, on the other hand, started to merge literally everything Libav did.

The reason for the fork is most likely that the developers hate each other. While this formulation seems somewhat sloppy, it is most likely the truth. To this date, the #libav-devel IRC channel still has Michael Niedermayer (the FFmpeg maintainer since 2004 according to Wikipedia) on their ban list (similar misbehavior is exhibited by some FFmpeg developers). There is little to no cooperation between the two projects.

More about FFmpeg's history and the fork incident can be found on Wikipedia.

FFmpeg has more features and slightly more active development than Libav, going by mailing list and commit volume. In particular, FFmpeg's features are a superset of Libav's features. This is because FFmpeg merges Libav's git master on a daily basis. Libav on the other hand seems to prefer to ignore FFmpeg development (with occasional cherry-picking of bug fixes and features).

Some Linux distributions, especially those that had Libav developers as FFmpeg package maintainers, replaced FFmpeg with Libav, while other distributions stick with FFmpeg. Application developers typically have to make sure their code works with both libraries. This can be trivial to hard, depending on the details. One larger problem is that the difference between the libraries makes it hard to keep up a consistent level of the user experience, since either library might silently or blatantly be not up to the task. It also encourages library users to implement some features themselves, rather than dealing with the library differences, or the question to which project to contribute.

FFmpeg and Libav developers also seem to have the tendency to ignore the damage their rivalry is causing. Apparently fighting out these issues on the users' backs is better than reconciling. This means everyone using these libraries either has to suffer from the differences, or reimplement functionality that is not the same between FFmpeg and Libav. mpv does not follow the latter approach to avoid bloat, thus the choice between FFmpeg or Libav matters.

Although mpv attempts to work well with both FFmpeg and Libav, FFmpeg is preferred in general. This is simply because FFmpeg merges from Libav, and seems to have more features and fewer bugs than Libav. Although we don't agree with everything FFmpeg does, and we like some of Libav's general goals and development directions, FFmpeg is just better from a practical point of view.

It shouldn't be forgotten that Libav is doing significant and important development, but since everything they do ends up in FFmpeg anyway, there is barely any reason to prefer Libav over FFmpeg from the user point of view. It's also possible that FFmpeg agrees faster to accept gross hacks to paint over bugs and issues than Libav, however, in the user's perception FFmpeg will perform better because of that.

This is pretty superficial, and should give only a general impression over differences. Also, this might be biased by the personal views of the author of this text.

• merges everything from Libav, so almost all Libav features and bug fixes are part of FFmpeg
• seems to be more popular (more patches, more activity on the mailing lists and the bug tracker)
• FFmpeg definitely has more features and more complete implementations of at least some file formats
• Libav has very long release cycles, which force us to be backwards compatible with very old code. Libav also likes to miss Debian/Ubuntu deadlines.
• Sometimes FFmpeg adds APIs we want to use, and Libav doesn't have them. This causes us pain, and the blame goes to Libav for not providing them.
• Really messy backporting of major FFmpeg features. (E.g. see VP9 decoder: after it was developed and merged in FFmpeg, Libav picked up the VP9 patches, made their own changes, both functional and cosmetic, squashed it into one commit. The original authors of the decoder had to figure out what was changed and possibly improved, and what was purely cosmetic.) (See also HEVC incident.)

• the insane amount and volume of merging from a codebase that has been diverging for several years most likely has some negative effects on FFmpeg
• FFmpeg is relatively paranoid about being compatible to Libav, which adds bloat, and also allows Libav to dictate the API, somewhat stifling FFmpeg development and adding stupid artifacts to the FFmpeg ABI
• Libav is going somewhat clean directions in API development (although everything from this ends up in FFmpeg anyway)
• FFmpeg seems to have an "anything goes" attitude, and merges/accepts just about anything, sometimes only for the purpose to have it before Libav. (E.g. see HEVC decoder: developed mainly on the Libav side, though separate from the Libav repository, it was hastily merged by FFmpeg shortly before Libav was done with the merge that was prepared for months.) (See also VP9 incident.)
• FFmpeg never cleans up anything. The shit keeps piling up.
• Libav does major work cleaning up the API and internals.

Libav lacks some features, and this list is supposed to give an overview which missing features have a larger impact on the features mpv appears to provide. Note that some of these features were removed from mpv's core and replaced with FFmpeg functionality. (In some cases, it turned out only later that Libav didn't provide the required functionality.)

• external vobsubs (.sub/.idx files) can be read only if built with FFmpeg (http://bugzilla.libav.org/show_bug.cgi?id=419)
• Libav misses support for a big number of various text subtitle formats (http://bugzilla.libav.org/show_bug.cgi?id=419) (mpv still has legacy parsers from mplayer, though)
• we plan to replace most video/audio filters with FFmpeg's, but Libav lacks some of them: vf_noise, vf_phase, vf_stereo3d, vf_pullup (some more MPlayer filters have been removed from mpv, but are available in ffmpeg's libavfilter)
• mpv can display ShoutCAST/Icy stream information on FFmpeg only (fixed in Libav 10)
• Libav has problems with badly interleaved AVI files (which means certain old AVI files might randomly desync after playing fine for some time) (http://bugzilla.libav.org/show_bug.cgi?id=666) (fixed in Libav 10.1 and possibly 9.x)
• Libav PGS subtitle decoder doesn't handle multiple subtitle rects (http://bugzilla.libav.org/show_bug.cgi?id=418)
• Libav http implementation doesn't return the content type, which makes opening web radio streams slow (plus mp3 streams might fail entirely to open) (fixed in Libav 10?)
• Libav doesn't support mid-stream ogg tag updates (popular with web radios) (http://bugzilla.libav.org/show_bug.cgi?id=668)

This list is not complete. In some cases we might not be aware of differences in functionality and performance until we encounter it. Most mpv developers exclusively use FFmpeg, which makes spotting such issues harder.

Wikipedia: http://en.wikipedia.org/wiki/FFmpeg#History