Only questions about programmatic use of the FFmpeg libraries, API, or tools are on topic
Libav (or libav*) is the collective name of the FFmpeg libraries: libavcodec, libavformat, libavfilter, libavutil, etc
"Not to be confused ffmpeg is more recent and libav was used in some distributions of linux"
from question "FFMPEG RTSP stream to MPEG4/H264 file using libx264"
"Ubuntu 12.04 ships with the ffmpeg fork libav in version 0.8 which is more compatible with ffmpeg 1.0+ or even later ffmpeg versions iirc"
from question "Avcodec_open segmentation fault"