Matcha is green tea leaves ground into powder form. Green tea leaves used to make matcha is Tencha. Tencha is a Japanese green tea which grown in a shaded farm just like Gyokuro (Gyokuro is the highest grade tea in Japanese Green tea). The post processing of Tencha is very similar to Gyokuro, except Tencha has no rolling process since it has to be ground to make matcha. Tencha in Japanese actually mean "ground tea" because in general Tencha is only used to make matcha.
International standard do not allow the use of the word "Matcha" if it's not made from Tencha. For green tea powder made with tea leaves other than Tencha, it should be labelled as "Green Tea Powder" instead of "Matcha".
On the other hand, matcha has much tighter quality requirement compare to normal green tea powder. Only green tea powder meeting criteria below can be called matcha
In most cases, there are 4 ways to differentiate between high grade and low grade matcha.
Matcha has already became part of Japanese daily life. Japanese consume average 1.1KG matcha per person per year. Japanese is known to have the longest life expectancy in the world, it's credited to matcha because of its health benefits.
Read about Matcha Misconceptions.