Think of the Hitch as two separate components (like Sam said)-- the Hitch
http://www.etrailer.com/Trailer-Hitc...eid=2009300532
Which looks like the above, is the thing being bolted on (or welded) to your Xterra.
That hitch will have two separate ratings--
a rating without a weight distribution system
and one with a weight distribution system
For example, there will be a sticker on the hitch which says:
Weight Carrying Capacity (without the weight distributor)
Gross Towing Weight (GTW):
4,000 lbs.
Tongue Weight (TW):
400 lbs.
Weight Distribution Capacity
Gross Towing Weight (GTW):
6,000 lbs.
Tongue Weight (TW):
600lbs.
That means if you buy a weight distribution system your Xterra can have
up to 600lbs on the pressing down on the hitch (tongue weight on the trailer) and that hitch (can pull
up to) 6000lbs. That doesn't change what your vehicle can do though it's just what that hitch is rated for

So that number is kind of worthess to you.
If you didn't get a weight distribution system and just went with a ball mount that hitch could take 400lbs on of tongue weight and could pull up to 4000lbs (again that has nothing to do with what your Xterra can do it's just what that hitch is rated for).
The second part of the system is what actually connects your trailer to your Xterra
http://www.etrailer.com/Weight-Distr...ese/66154.html
(just an example of one)
All that stuff connects to your shank which goes into your hitch.
They call it a weight distribution system because what it does is takes the downward weight from the trailer and instead of all that weight pushing down on the rear of your SUV it distributes some of that weight onto the front axes of your SUV.
Here is a video--
that can explain how it works better then I can.
Hope that helps