Africa is a continent long in the north-south direction. Various terms are used to refer to parts of it. There's "Sub-Saharan Africa", meaning Africa south of the Sahara desert, south of the Muslim and Arab world.
"Southern Africa" is a geographic region, and a less-well defined term, even if it is commonly used. It logically must include South Africa, right at the south end (must therefore also include the small kingdoms of Swaziland and Lesotho), but must include more than that to have a distinct meaning. Then there are the states that border on South Africa: Namibia, Botswana, Zimbabwe and Mozambique, and the island of Madagascar.
The next layer north of these are Angola, Malawi, Zambia and the north end of Mozambique again.
After that you're into The Democratic Republic of the Congo, Rwanda, Burundi and Tanzania, but this is stating to look like Central Africa now.
Or a more political, less geographic definition: The Southern African Development Community (SADC) has the following member states: Angola, Botswana, Democratic Republic of the Congo (DRC), Lesotho, Malawi, Mauritius, Mozambique, Namibia, Seychelles, South Africa, Swaziland, United Republic of Tanzania, Zambia and Zimbabwe.
Or you could just take eveything south of the equator, a line that goes through Gabon, Congo, Zaire, Uganda, Kenya and the south end of Somalia. I would however prefer a subtropical dividing line.