If you only use them for listening to music, get stereo. If you will be watching movies with them on or playing games with them, you may want surround 5.1 or 7.1 headsets.
Stereo is the classic Left & Right(Two sources of audio channels)
5.1 has Front Left, Front Right + Surround Left, Surround Right, a Center channel and a Sub Woofer(.1)
7.1 has all of the above plus two more channels Side Left and Side Right
As mentioned, the "surround" is simulated in most headphones and some do it better than others. Human hearing is binaural so sometimes the additional channels crowds out what your are trying to listen to. I prefer to use my Sennheiser 558's over my "gaming" 7.1 headphones for nearly everything... including games.