Okay, so, visas for Africa can sometimes be a tad complicated, depending on where you plan to travel. This information applies to UK residents, and while we'll try and be as thorough as possible, it's definitely worth speaking with your travel agent to find out exactly what documents you need for where.
Africa is divided into four customs unions that have different policies, and these can further vary from country to country.
Southern Africa: South Africa, Botswana, Lesotho, Swaziland.
Western Africa: Senegal, Guinea-Bissau, Mali, Burkina Faso, Cote D'Ivoire, Benin, Togo, Ghana, Niger, Gambia, Guinea, Nigeria, Liberia, Sierra Leone.
Central Africa: Cameroon, Gabon, Central African Republic, Chad, Equatorial Guinea, Republic of the Congo
East Africa: Kenya, Uganda, Burundi, Tanzania, Rwanda
You will usually need to get individual visas for these countries, even if they're in the same union, but it's easier to get a visa for one of these countries if you're already inside the union. If you're in West Africa you can get a Visa Touristique Entente, which for a relatively cheap price covers Benin, Burkina Faso, Niger, Togo, and Cote d'Ivoire, valid for two months.
If you're going on a short trip to only a few African countries, and you have a set itinerary, it's usually possible to get all your visas before you travel. If you're planning to go backpacking for longer, and want a little more flexibility, it'll probably be easier to get them as you go along. Most countries will have an embassy in each neighbouring country, but this isn't always the case, so some planning is definitely required.
It's also worth researching where you'll need a visa. British citizens don't need advance visas for many southern African countries. Cost will also depend on what passport is in your hand.
You also need to consider that some countries don't have particularly stringent rules, and there are times where a bribe can go a long way. Not that we condone that kind of behaviour, of course.
Travel in Africa can be a complicated business, especially as things can change rapidly, so doing some research beforehand, or speaking to other travellers on the road, is definitely recommended.