Not exactly common because of the by-hand processing involved, oolong tea can be expensive. It is known for its complex fragrances and tastes.
Gentle rolling after picking allows a slow oxidation that is carefully regulated to produce the desired effects, having to take into account such things as weather conditions, leaf quality, and duration of oxidation. This partial oxidation causes oolong teas to be called "semi-fermented" teas (oxidation and fermentation are synonymous in reference to teas).
Oolong tea has a little less caffeine than black tea.