There are several types of tea; Black, White, Yellow, Green, Oolong and the famous Pu-erh.
All come from the Camellia Sinensis plant, but the leaves are treated slightly differently. Black tea leaves are allowed to oxidise, which means that they react to the Oxygen in the air and turn slightly brown.
Green teas are not subject to the same process, instead being heat treated before this happens, which halts the process.