Tea is only made from the leaves of Camellia sinensis, not other ornamental camellias. All tea, whether it's green, black or oolong, comes from this one plant species.
I like to think of tea like I do potatoes. There are many things you can do with one potato, it's the way you process that makes what you get. For example, if you want mashed potatoes, French fries, or potato chips, you do different things to it.
Tea is just the same. Different types of tea goes through different processing methods to bring you the different types of tea. Black Tea is tea that has been allowed to oxidize and ferment, much like a banana turns brown when it's old or bruised. When the cell walls of tea leaves are bruised, it causes a chemical reaction that gives you the wonder taste and aroma of rich black tea.
On the other hand, green tea is just that....Green. It goes through no oxidation process and thus does not ferment or turn brown.
The different types of tea go through different levels of withering, bruising, and firing or heating on their journey to their final destination.