Converting Video Formats Using ffmpeg on Linux:

Installing ffmpeg on Fedora 12,13,14:
In fedora 12,13 and 14 first of all you will need to enable access to rpmfusion free and non free repositories using the following command:

# su -c 'yum localinstall --nogpgcheck http://download1.rpmfusion.org/free/fedora/rpmfusion-free-release-stable.noarch.rpm http://download1.rpmfusion.org/nonfree/fedora/rpmfusion-nonfree-release-stable.noarch.rpm'

and then running command:

#yum -y install ffmpeg

will install ffmpeg on your fedora machine.
Installing ffmpeg on Ubuntu 10.x/Linux Mint:

sudo apt-get install ffmpeg libavdevice52 libavfilter0 libavcodec-extra-52 libavformat-extra-52

Sample Commands:
Simple conversion with ffmpeg defaults:

ffmpeg -i inputfile.format outputfile.format  

Changing frame rate:

ffmpeg -i input.avi  -r 24 output.avi 

Converting to mp4 using mpeg4 codec

ffmpeg -i input.flv -vcodec mpeg4 -acodec copy output.mp4

Converting video for  cell phone :)

ffmpeg -i input.format -s qcif -vcodec mpeg4 -r 18 -acodec copy output.mp4

Extracting audio in mp3 format:

ffmpeg -i input.avi -vn -acodec copy output.mp3