From the World Heritage inscription:
This unique archaeological and ethnological reserve, located in the Northern Territory, has been inhabited continuously for more than 40,000 years. The cave paintings, rock carvings and archaeological sites record the skills and way of life of the region’s inhabitants, from the hunter-gatherers of prehistoric times to the Aboriginal people still living there. It is a unique example of a complex of ecosystems, including tidal flats, floodplains, lowlands and plateau, and provides a habitat for a wide range of rare or endemic species of plants and animals.
Kakadu doesn’t get as much attention as other sites in Australia, but it is arguably the best. Kakadu is home not only to more wildlife than you will find almost anywhere else in Australia, but is also the home to some of the oldest rock paintings on the continent.
Kakadu is a 2-3 hour drive from Darwin in the Northern Territory. In addition to saltwater crocodiles and kangaroos, you can see stunning waterfalls and enormous termite mounds.
Much of what you experience will be determined by the time of year you visit. I visited at the beginning of the dry season, which was a great time to view crocs on the river, but not so great for going to the waterfalls (the roads were closed).
Many of the outback scenes from the movie Crocodile Dundee were shot in Kakadu.