Matcha is powdered green tea, traditionally used in Japanese tea ceremonies. In order to increase the flavor and color of the tea, the tea plants used for matcha are cultivated in the shade for about two weeks before harvesting. This stresses the plants and they produce more caffeine and theanine, a brain calming amino acid.
The tea leaves used for matcha are called tencha. Darker green tencha is used to make ceremonial (higher) grade matcha and the lighter green is used to culinary matcha. Recently, there's been a "matcha boom" in the US as health conscious consumers add it to their lattes, smoothies, and baked goods.