Oolong tea is a semi-oxidized tea. It is often considered to be somewhere in between Green and Black tea, with both light and dark notes. Different types of Oolongs vary in the level of oxidation that they receive.
Traditional styled Oolongs tend to be more oxidized than modern style Oolongs. The modern style of oxidizing Oolongs is focused on preserving the bright fresh lightness of the leaves. The modern style preserves the greenness in the finished tea. Though lighter Oolongs are meant to preserve a freshness in a similar way to Green tea, leaves meant for Oolongs are bruised before being fixed. This bruising promotes oxidation and withering, that allows the leaves to oxidize slowly. Oolong is also one of the best teas to roast. Roasting Oolong promotes smoothness in the darker notes of the tea. Oolongs typically have a buttery sweet aroma when processed in the modern style, and a deeper buttery and charcoal aroma in the tradition style and when roasted. Oolongs are known for coming rolled, in the shape of a ball or in strips similar to Black tea.
Oolong teas are most famously produced in China and Taiwan. The methods of production have mutually effected the production in both of these places. One of the most noted diversions was between the modern and traditional styles. However, the different methods are practiced in both places today.