\Like anything else, it depends on where you live.
GVWR- Gross Vehicle Weight Rating, including the vehicle and all occupants and cargo IN the vehicle. For a pickup, that would be the total weight of having scales under all 4 wheels with the driver in the truck.
GCWR: Gross Combined Weight Rating, including the vehicle, all occupants and cargo, and the trailer (including cargo on the trailer). GCWR is different than the GVW, and can usually be found in the owner's manual or on the manufacturer's web site. This, again, depends on where you live- some places (like N.H.) won't allow you to tow any trailer above the GCWR. Other states allow you to tow anything as long as you are registered to tow that much.
Keep in mind that the two numbers do affect each other. Lets say you have a 5000 lb. truck (empty), an 8000 lb. GVW, and a 12000 GCWR. If your truck is empty, you can hook up a 7000 lb. trailer and be legal. But if you put 3000 lbs. in the bed of the truck, you can only tow a 4000 lb. trailer.
And one more thing- the trailer tongue weight does go against the GVW of the vehicle. In other words, at NO TIME can the weight of the vehicle (truck itself on a set of scales) go over the GVW. If you have a trailer hooked up and the DOT decides to put you on the scales, you will be weighed with the trailer attached, so remember that when loading the bed of the truck.
Although, once again, YMMV depending on where you live.