Matcha, if you have never heard of it before, is a strikingly coloured green tea powder from Japan. Unlike most teas, which you can prepare by soaking a teabag in water, matcha is meant to be stirred in and dissolved in the water.
Matcha is a little bit like hot chocolate mix, except much greener and without the taste of hot chocolate. I guess the only similarity really is that both powders are soluble in water.