The term “tea tree” is actually used as a common name for plants in a wide variety of genera, with no real common traits. These trees grow around the world and produce foliage that is brewed into tea and pressed to yield naturally antibacterial oils. They are also used as firewood and as ornamental plants. The general uncertainty surrounding the term can get rather frustrating, especially there are significant differences between the species. It's often used for plants in the Melaleuca genus, which produce an oil used topically for its antiseptic properties.
The true tea plant is Camellia sinensis, which literally yields tea, the caffeinated beverage that is brewed from the leaves after they are specially treated. All true teas are made with the leaves of this plant, while hot beverages made from other plants are known as tisanes or infusions. Most people have consumed this beverage at some point, as it is immensely popular around the world.
People also use the term “tea tree” to talk about plants in the Melaleuca genus, which are native to Australia. These plants are characterized by prickly needles and bark that is often papery. The needles of the tree produce a highly aromatic natural oil that has antibacterial and antifungal properties. Native Australians have used the plant in medical treatment for centuries, and when early explorers first began adventuring in Australia, the tree was one of the first things they learned about. Captain Cook is responsible for the common name of the plant, because he and his crew used the leaves to brew a form of tisane.
Plants in the Leptospermum genus, which are native to Australia and New Zealand, are also sometimes called "tea tree." These trees also have needly leaves, and they produce some interesting compounds of their own. Another common name for plants in this genus is “manuca,” and consumers can find various products that include it on the market, especially in New Zealand. Kunzea ericoides, another New Zealand plant that was once classified in the Leptospermum genus, is also sometimes called “white tea tree,” in a reference to its abundant small white flowers.
Ornamental boxthorns are sometimes called tea trees, although these plants share nothing with the above plants except for the common name. These plants can often be quite pretty, with delicate flowers and lush foliage, but they are also rather savage plants, with nasty thorns that can make a tumble into a hedge very unpleasant.