First wort hopping, as I understand it, means adding the bittering hops in the boil kettle when your running off the mash.
From what I've read, some people claim that it gives the beer a smoother bitterness. Others aren't so sure. I don't know anyone that's done a side to side comparison of the two, or tried brewing using FWH vs regular hopping.
Something of interest that I've read recently regarding hopping and boil time: A 90 minute boil is used, where you start adding your bittering hops at 60 minutes. The first 30 minutes of boil allows for hot break to form and precipitate out....meaning it's not in the wort to glom on your hops.