Black tea leaves will go through more processes than green tea leaves.
After the leaves have been withered they are then broken up or rolled and laid out exposed to the air. This allows the natural juices, or enzymes in the leaves, to be released and, on contact with the air, they will oxidise.
After a few hours the leaves turn a golden russet colour and oxidisation is complete. The leaf will then be dried and all the moisture is evaporated and the leaf turns a dark brown or black and we have that recognizable, strong, flavour and aroma. Of course black teas are not all the same - the differences lie in the country, region and conditions in which they are grown.
This process does mean that black leaf tea is slightly higher in caffeine than either green or white tea and that less of the natural health benefits of the leaves are retained, but in return we have this great amber brew.