Difference between a serving keg and a fermenting keg?

[xmacro]

I've seen it on a few websites, namely kegglebrewing.com and I'm wondering what the difference is, if any?

 

[beaksnbeer]

The racetrack/ center opening is drilled to take an airlock.

 

[Catmandu]

Some folks also cut the dip tube a little shorter in the fermenting keg to help avoid transferring trub to the serving keg.