Oolong teas are semi-oxidized teas. They are sometimes called 'semi-fermented teas,' though this moniker is considered to be less technically correct than semi-oxidized teas. After the tealeaves are picked, they are rolled and allowed to oxidize. Oxidation produces the floral notes that characterize many oolongs.
Often, oolongs are alternately rolled and shaped, which allows for more complex, nuanced flavors, more controlled oxidation and more intricate shaping. Common shapes for oolong teas include semi-ball or ball shapes and wiry, twisted shapes. Balled and semi-balled oolongs are best enjoyed over many infusions, preferably in yi xing teaware with gong fu-style brewing.
After oxidation and partial shaping, oolongs are heated to halt oxidation and carefully shaped one last time. Beyond this basic heating, many oolongs are roasted. The roasting process can give oolongs darker aromas and flavors, akin to ripe fruits (especially stone fruits), nuts, roasted grains, caramel, coffee or chocolate.
The range in oxidation, shaping and roasting makes oolong tea a broad category of tea with an enormous span of flavors and aromas stretching from fresh, clean and grassy / vegetal to dark, roasty, fruity and even espresso-like. Because they are so lightly oxidized, some people separate 'Baozhong' or 'Pouchong' oolong teas into a separate category of 'green oolong' tea.
Most oolong teas hail from China (especially from Fujian Province and the Wuyi Mountain Range) and from Taiwan. These regions are particularly famous for their skillfully handmade oolong teas. However, in recent years, other countries (notably India and Sri Lanka) have begun to produce oolong tea, usually on a large scale in a factory setting.
In Taiwan, some oolongs are aged for many years for a more refined, subtle flavor.