Say that you want to cut out a part starting at 00:00:30 into the original file with a 5 seconds length and that it shouldn't be re-encoded (if you want to re-encode it you'll have to replace copy with audio and video codecs, for a list of available codecs issue ffmpeg -formats -E), then you issue:
ffmpeg -ss 00:00:30 -i orginalfile -t 00:00:05 -vcodec copy -acodec copy newfile
You'll have to replace orginalfile and newfile with actual file names, the later is the name of the file that is created to hold the cut out part.
OR - to set partial second start and shorter end, while dropping video and recode audio to MP3: