Basically there are two species of African rhino - the black and the white. White rhinos can be divided into two sub-species. The Southern White and the Northern White. The latter are now thought to be extinct. There were recently a few in a remote part of the Democratic Republic of Congo, but despite a courageous effort to save them by the late, great Lawrence Anthony who quite literally put his life on the line to save the animals, the last few Northern White Rhinos on earth were taken by poachers amid the chaos created by corrupt governments and the Lord's Resistance Army. I thoroughly recommend that you read Lawrence Anthony's book The Last Rhinos.
So what's the difference between a black rhino and a white rhino? It's not as obvious as it seems. Both are grey actually, or the colour of whatever dust or mud they've been rolling in. "White" rhino springs from the dutch word weit meaning wide. It refers to the animals mouth which is indeed wide. Black rhino's have a kind of prehensile hooked lip, perfect for curling around the branches and twigs of the low shrubs and trees that form the greater part of it's diet. The white rhino is perfectly adapted to munching on grass. Basically, it is an armoured lawn mower. They hold their head lower and consume grass by the kilo. It's quite easy to tell which animal has passed by when you look at their droppings. White rhino dung is almost exclusively grass, while a close inspection of black rhino dung reveals leaves and twigs all chomped at a forty five degree angle. African dung is fascinating.
There are other differences too. Black rhinos are much more aggressive and secretive despite being smaller than their white cousins, although white rhino cows can be very dangerous if they have calves. They are even different in the way they retreat from threats. White rhino cows will run away behind their calves, thus protecting their offspring from threats from behind as they flee across the grassy savannah. Black rhinos will run away ahead of their calves, clearing a way through the scrubby habitat that they favour.
Everyone knows that the rhino is one of the "Big Five" along with the elephant, the lion, the leopard and the cape buffalo. They were originally so called because they are the most dangerous animals to hunt, but actually you haven't officially seen the Big Five unless you've seen a black rhino. However, since they are not easy to find we'll forgive you for claiming the big five if you've seen a rhino of either kind.
Anyway, if you really want to help to preserve these beautiful animals, please visit Africa. Go and see them for yourself. Not only will you enjoy a life changing adventure and a wonderful holiday, but you will reinforce the fact that a live rhino is far more valuable as a sustainable resource than a dead one with its horn hacked off.