In the West, the more common meaning for "red tea" is rooibos, which is actually a type of tisane or herbal infusion. Rooibos is a naturally caffeine-free herbal "tea" with a red color and a sweet, woodsy, earthy flavor that some say is similar to black tea. Due to its sweet flavor, rooibos is used in many dessert teas and masala chai blends.
There is also a much older meaning for "red tea," one that originated in China several hundred years ago. It is synonymous with what people in the West call "black tea."
Hong cha, or "red tea," is the original name for "black tea" (hei cha) and it is used in the East because, in China, teas sometimes are named for the color of their infusions. In the West, teas are sometimes named for the color of their leaves. In the cases of white tea and green tea, the color of the leaves and the color of the infusion is fairly similar, so the names are the same. However, black / red tea tends to have dark leaves that are nearly black and a much lighter, much more red infusion. In Chinese tea terminology, black tea (hei cha) is a completely different type of tea whose processing is characterized by fermentation rather that oxidation.
Black / red teas are noted for their bold flavors, which may be tannic, earthy, sweet, woodsy, floral, fruity or chocolaty. (For more information, see how to taste tea and tea flavor profiles.) Due to their bold flavors, black teas pair well with many foods, and they are a popular choice for afternoon tea.