To convert the entire video to GIF, use the following command:
ffmpeg -i small.mp4 small.gif
To convert just a portion of a video clip to GIF, use the following command:
ffmpeg -t 3 -ss 00:00:02 -i small.webm small-clip.gif
The snippet above directs ffmpeg to create a GIF 3 seconds long starting at 2 seconds into the video. The default conversion doesn't appear to be high quality, so you can configure the bitrate via another parameter:
ffmpeg -i small.mp4 -b 2048k small.gif
The command is quite simple:
ffmpeg -f gif -i animation.gif animation.mp4
You can use this same command format to convert to other video formats:
ffmpeg -f gif -i animation.gif animation.mpeg ffmpeg -f gif -i animation.gif animation.webm
ffmpeg and ImageMagick are awesome media utilities which you should take some time to check out if you have any questions about how to get something done!
gifsicle to scale down the image to a smaller size
ffmpeg -i foo.mp4 -r 5 -vf scale=270:-1 foo.gif , where -r 5 cuts it to 5 frames per second, and
-vf scale=270:-1 scales the output to a width of 270 pixels and a height that matches the aspect ratio.