Everything Michael said about matcha was completely correct. I hope he doesn't mind me adding in my own comment, though. I've take Japanese tea ceremony lessons and drink matcha fairly regularly. There are two varieties of matcha, usucha (thin tea) and koicha (thick tea). If you are new to matcha, I would suggest usucha; in chanoyu, the first ceremony you learn is the usucha ceremony. As Michael said, it is indeed best to purchase matcha directly from Japan. I would suggest http://www.teatoys.com/kmatcha.html
. The latter is where I would recommend for those new to matcha, as you can also get a whisk and tea bowl to make and drink your matcha. There are a few places in the United States where you can take chanoyu lessons: New York, Philadelphia, Boston, San Francisco, and other scattered locations. Hope that helps.