At first there was only FFmpeg, but in 2011 its leader forked it into libav and used it by default on Debian. Both the two multimedia frameworks for manipulating media files do the same tasks.
Starting with Ubuntu 15.04 Vivid Vervet, FFmpeg was included in the default repositories replacing Libav and will be used on Debian Unstable and Ubuntu 15.10 as default (again), because the Debian maintainers consider that Libav is patched to slow compared to FFmpeg.
Recently, Michael Niedermayer, the current leader of FFmpeg has announced that he is resigning from his function hoping that the Libav and FFmpeg will get reunited again.
This is from Michael’s resignation letter:
“The community is split and its very difficult to be the leader when one is on one side of this split and the other tries everything to push me out I hope my resignation will make it easier for the teams to find back together and avoid a more complete split which would otherwise be the result sooner or later as the trees diverge and merging all improvements becomes too difficult for me to do.”