ffmpeg is for video stuff. The basic principle is to take a video/audio input, and transform it to a video/audio output:

ffmpeg -i <input> <output>

Trimming a Video

A video can be trimmed using the following pattern:

ffmpeg -ss 00:00:40 -to 00:01:16 -i input.mp4 output.mp4

-ss is the start time of the clip you wish to cut, -to is the end time of the clip, and -i is the input video file.

There are more accurate ways to cut video using ffmpeg, but this works fine for my purposes.

Screen Recording and Webcam

Thanks Luke.

Screen Recording

This records both screens into out.mp4. Here the input :0.0 just means the screen, because we specified the format x11grab.

ffmpeg -f x11grab -i :0.0 out.mp4

You may need the -s tag for the screen size (and this should go before the input -i).

Webcam

Here the input is

ffmpeg -i /dev/video0 out.mkv 

To add audio (from pulseaudio),

ffmpeg -i /dev/video0 -f alsa -i default out.mkv