All of these types of tea actually depend on the levels of oxidation. Black, Green, White and Oolong are all harvested from the flowering plant, Camellia Sinensis, and the type of tea is determined by the different levels of oxidation. Black tea is oxidised the most, then Oolong, then Green, then White with the least.
In my opinion I think there are more than just the four categories of tea. Other companies (like Teavana) sell branches of different tea like maté, roobios and herbal tea. But according to "the professionals" it will not be considered true tea unless it comes from the Camellia Sinensis and is prepared in the standard way. To me tea is tea, and it's good. But if you want to be technical, beverages like roobios or maté are considered 'tea-infusions'. Another 'borderline tea' is pu-erh which does come from the Camellia Sinensis but is prepared using a different method called post fermentation process. What do you think, are there only four categories of tea or can there be more? Let me know in the comments.