All tea comes from the same plant (camellia sinensis) and like most plants, its leaves are green. Green tea preserves that green color and freshness. China and Japan produce the highest quality green teas, though many other countries are now producing it as well. Green tea usually produces a light body with mild astringency and a vegetal/grassy character.
Matcha teas are a distinct style of Japanese green tea that is different in every aspect to all other teas. Matcha tea is shade grown tea (e.g., Gyokuro, Kabusecha), with the leaf matter removed from the fibrous veins and stems. This leaf material (called Tencha) is then milled between two stones until it is a fine powder called Matcha.
The Matcha is then whisked into hot water where the liquor and leaf are both consumed. It is not steeped and removed from the water like traditional teas. Matcha is the tea served during the famous Japanese tea ceremony, but culinary grades are now used for all kinds of more casual purposes such as smoothies or baking.