The hitch is the part that is bolted onto the truck frame, with a square (2" for Class IV) receiver to accept a shank or ball mount. The hitch is classified by how much weight it can carry. A class IV hitch can carry up to 10000 lbs.
That doesn't mean you can tow a 10000 pound trailer because tow capacity is governed by the truck specifications (power, tranny, suspension, brakes, etc.).
A ball mount with a hitch ball is inserted into the receiver hitch. A weight-distribution system is actually installed onto the trailer A-frame and onto a special type of ball mount. The WD system usually consists of spring bars and chains (for adjustment) that transfer the tongue weight of the trailer to the front axle of the tow vehicle and the trailer axle(s).
So, with the Big Tow package, all you get is the Class IV hitch. If you plan to tow a trailer smaller that 5000 lbs., all you need is a standard ball mount and ball, hook up the trailer (with lights of course) and away you go. If you plan to tow a trailer larger that 5000 lbs. (like me), you need to buy a ball and special ball mount for the truck and a WD system for the trailer.
Take a look at the following websites for examples... www.reeseprod.com
and look under "products" and then "WD systems."
Hope this helps...
Let me know if you have other questions.