The Masai Mara is the largest game reserve in all of Kenya. It is actually the northern extension of the great Serengeti ecosystem, spanning the midwestern section of Tanzania and northward across the Kenyan border into Kenya. Each year, both the Serengeti and Masai Mara experience the migration of millions of Wildebeest and Zebra from the south lands to the north lands and back again. The herds follow the rains and good grazing.
Wherever you have large numbers of prey animals you also find large numbers of predators. There are Lions, Leopards and Cheetah in abundance and all make use of the migration to insure their own survival. The "clean-up crews" (Hyenas, Jackals, Vultures, etc) are equally as plentiful.
The Mara is restricted in that the govt only allows a certain number of guest lodges to maintain a presence in the reserve, therefore the number of tourists is somewhat controlled.
A tracker or guide is virtually a necessity in a place like this. They do know where the animals "hang out" and in an area as large as the Mara, you can drive for days and see nothing, unless you know where they are.
You can, however, drive your own vehicle in some parks, if you feel you would like to do so, although it is not advisable.