SmokeyBarnable's excellent write-up leaves out a couple of details I'd like to add.
First of all, in case you're curious as to why people don't just dig holes in the ground and defecate into them, it's usually because that particular piece of land sees way too many visitors to process all that excrement naturally. Rafting enthusiasts encounter these situations more often than most, since they're all going to stop at more or less the same spots along the river to eat, crap, and sleep. Packing out everything (including feces) substantially reduces the environmental impact on these heavily used campsites.
In addition to the toilet seat upgrade, groovers have evolved past the rocket box into some relatively luxurious models. In his book, Up Shit Creek : A Collection of Horrifyingly True Wilderness Toilet Misadventures, author Joe Lindsay describes the Jon-ny Partner and a deluxe groover nicknamed the Millenium Falcon, both of which are made of stainless steel and allow easy access for bag changes. Even with lesser models though, the telltale groove marks that gave the groover its name are largely a thing of the past.