Formosan teas are usually Oolongs, which are semi-fermented (half way between a fully fermented black tea and an unfermented green tea).
Oolongs grown in Taiwan account for about 20% of world production. A government Tea Inspection Office grades teas into 18 categories ranging from Standard to Choice
They are incredibly refreshing, on a hot day they revive you in a way other drinks can't. You would not add milk to them but a slice of orange would not be too decadent.
Oolong is harvested five times per year in Taiwan, between April and December, with the July and August crops generally receiving the highest grades, rather than the April first flushes.