Not actually a part of the Serengeti National Park, Ngorongoro Crater is in the Ngorongoro Conservation Area, adjacent to the Serengeti. Ngorongoro is unique both in terms of the land as well as the animals. Called a crater, Ngorongoro is actually a caldera. At 18km across, it is the largest unflooded and intact caldera in the world! The animals are unique as well. While not an isolated ecosystem, there are many animals in the crater, some live there permanently. This is due to the permanent water source. The lions especially, have been shown to be extremely inbred.
Ngorongoro Crater is also known as a great place to see elephants. There is a herd of elephants in the crater, and they are all very impressive to see. It is a bachelor herd, made up of old male elephants - driven away from their herds when they got old. The elephant families stay in the highland forests around the crater, and do not often venture into the crater. Males leave their herds at 12 years old or older, associating only with other bulls from then on (except when mating is an option).
While Ngorongoro is not by any means independent from the rest of the world (animals come and go regularly - meeting an elephant who is leaving the crater while you are entering the crater can be a harrowing experience!) it is somewhat isolated. This, combined with the amount of water always in the crater has made Ngorongoro an attractive place for animals of all sorts (including tourists).
There are several lodges on the rim of the crater, which rises to around 2300 meters. They range from the (normal, 3 star) Wildlife Lodge to the (insanely expensive and rediculously posh) Ngorongoro Crater Lodge, with chefs who are reknowned in New York and Paris.
Ngorongoro Crater is normally visited en-route to the Serengeti, from Arusha or Nairobi. It is one of the premier tourist locations in Tanzania. All in all, Ngorongoro is a beautiful place with amazing wildlife that is worth visiting if at all possible!