In addition to having great flexibility over demuxer, decoder, processer, encoder, muxer choice and settings, ffmpeg can crop, stream, merge audio and video from different sources and perform many other tasks.
Because ffmpeg allows transcoding on the fly and supports multiple streaming protocols such as rtmp, rtsp, http, ftp, hls, you can use it to stream to services such as twitch.tv or set up your own streaming solution.
You can use both local realtime recordings or another stream as a source, transcode it if necessary, and output it to a different stream.
ffmpeg -i rtmp://server/live/originalStream -c:a copy -c:v libx264 -vpre slow -f flv rtmp://server/live/h264Stream
Convert videos to 159 formats including MP4, MOV, 4K, 3D, MP3, AC3, etc.
Convert videos that suit for iPhone, Samsung, Sony, Xbox, PSP, Archos, Roku etc.
Burn videos to ISO files, DVD discs, or DVD folders
Include basic editing function: merge, trim, crop, watermark, Subtitle, etc.
Download video from YouTube, Facebook, Yahoo, Vimeo and others
Stream videos from PC to TV wirelessly