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-vn removes tha video content, the copy option tells ffmpeg to use the same codec for generating the output
I used an flv in my example, but it'll work on any file ffmpeg supports. It says it wants an output file, but it tells what you want to know without one.
This converts all m4a files in a dir to flv. You can just swap the m4a bit to anything else ffmpeg supports though, and it'll work.
This is handy for making screenshots of all your videos for referring to in your flv player.
Download YouTube videos as .flv and convert them to .3gp for your mobile phone.
-vcodec, you choose what video codec the new file should be encoded with. Run ffmpeg -formats E to list all available video and audio encoders and file formats.
copy, you choose the video encoder that just copies the file.
-acodec, you choose what audio codec the new file should be encoded with.
copy, you choose the audio encoder that just copies the file.
-i originalfile, you provide the filename of the original file to ffmpeg
-ss 00:01:30, you choose the starting time on the original file in this case 1 min and 30 seconds into the film
-t 0:0:20, you choose the length of the new film
newfile, you choose the name of the file created.
Here is more information of how to use ffmpeg: