抹茶, matcha, literally "powder tea", is made from shade-grown tea leaves also used to make gyokuro (玉露, the finest green tea) and it's most commonly used in a traditional japanese tea ceremony. Its incredibly green colour is a result of covering the tea bushes from the direct sunlight for at least 2 weeks. After harvesting, if the leaves are rolled out before drying as usual, the result will be gyokuro tea. However, if the leaves are laid out flat to dry, they will crumble and become known as tencha (碾茶), from which matcha is made.

You can buy it in good tea stores, in some supermarkets, and it's easily available online. But before you buy it, I stronly advise to ask about the storing conditions - because all japanese teas, but matcha especially, needs to be kept cold, to maintain the freshness, color and flavor. At home we should keep it in refrigerator.

I strongly recommend to try out matcha, and experiment with it on your favourite desserts! You'll be surprised how well it tastes... I'll be posting some recipes for you as well :)