The black tribes in South Africa are divided into four major ethnic divisions - Nguni, Sotho-Tswana, Shangaan-Tsonga and Venda. The Nguni is divided into four groups - Zulu, Xhosa, Swazi and the Ndebele. They represent about two-thirds of South Africa’s Black population.
The history of the Nguni people is captured in their oral tradition. They were a group of people who migrated from Egypt to the Great Lakes region of sub-equatorial Central/East Africa. They then followed the eastern part of southern Africa in their southward bound. About 2,000 years ago they entered what is now South Africa. Another larger wave of migration followed around 1400 AD. This migration happened about a 100 years before the Europeans rounded the Cape of Good Hope.
As they start to settle the Southern Africa, the Nguni people split off into the different group known today - the Swazi in the north, the Zulu towards the east and the Xhosa in the south.
Owing to the fact that these people had a common origin, their languages and cultures show marked similarities. At some point along their southward journey, they came in contact with San hunters Khoikhoi herders, which they conquered and absorbed the women and children into their tribes. From Khoikhoi and San, they inherit the "click" sounds that characterise their languages today.
Sources:
http://www.mwelase-clan.com/45302121
http://www.sahistory.org.za/article/ndebele