There are two languages called Ndebele: Southern Ndebele and Northern Ndebele. Both are part of the Nguni group of Bantu languages, but they are not very closely related.
South Ndebele (isiNdebele) is spoken by about 590,000 people in South Africa, where it is one of the official languages. There are two dialects: Northern Transvaal Ndebele (Nrebele) and the Southern Transvaal Ndebele.
Northern Ndebele (Sindebele) is spoken in Zimbabwe and Botswana by about 1.5 million people. It is closely related to Zulu and mutually intelligible with it to some extent.