Sometimes I want an easy way to cut off some parts of a video. I don't want to use a full-fledged movie editing software for a simple thing like that and I don't like having to configure any output encoding settings. ffmpeg offers a nice solution:
$ ffmpeg -ss 00:03:33.0 -t 00:12:04.0 -i input.mp4 -acodec copy -vcodec copy -async 1 output.mp4
Time format is hh:mm:ss.x. The output video will start at 3min33s and end after 12min4s at 15min37s.
The -acodec copy and -vcodec copy parameters ensure that the audio and video codec settings used are copied from the input video.
If you only want to cut off a part in the beginning, just leave away the -t option.