glass mount attaches to your windscreen usually behind the mirror! roof mount goes on your roof, both do the same job!
Well - they do.
But ...
The glass mount will invariably get a worse reception than the roof mount. The glass mount is lower for one, and the roof mount will earth off the car frame which will make sure the reception is better.
I've also heard reports of the glass mounts coming off at high speed if they aren't attached properly.