The Serengeti is an aptly named huge plain in northern Tanzania (its Swahili meaning is 'Endless plains'). It's bordered to the east by the Great Rift Valley, along with several volcanoes like Mount Meru and Ngorongoro. To the west lies the vast Lake Victoria. The plains extend up north into Kenya, where they're called Masai Mara.
The treeless plains may not look like a hospitable habitat but it's populated with wildlife.
Most people know the Serengeti as a wildlife reserve and for a reason; in the Serengeti you can see about every animal there's to see in east Africa, including the Big Five: (Elephant, Lion, Rhinoceros, Leopard and Buffalo). Next to these there are cheetahs, crocodile, all kinds of antelopes, and in may you can see the wildebeest gather for their trek west towards the water.
Safaris towards the Serengeti mostly start in Arusha and are often combined with visits to Ngorongoro National Park and Lake Manyara National Park.