Black teas are generally fully oxidized and green teas are unoxidized, so many people say oolong is "between black and green tea" in terms of appearance, aroma and flavor.
The amount of oxidation relates to the type of oolong. Lightly oxidized oolongs are Bao Zhong Oolongs, like the one pictured above. They have flavors that are more similar to green tea than black tea, but they are also very floral in aroma and in taste. Because they are so lightly oxidized, some consider Bao Zhong teas to be a separate category of tea from oolong tea.
Semi-Balled Oolongs (such as Tieguanyin/Tie Kwan Yin, Formosa Oolong, High Mountain Oolong, Jade Oolong, Four Seasons Oolong and Dong Ding / Tung Ting Oolong) are more oxidized than Bao Zhongs. They have full, floral, creamy flavors and aromas. Sometimes, they are more heavily roasted or roasted repeatedly for a smoother, deeper, more woodsy/roasty flavor.
Oriental Beauty Oolong (a.k.a. Bai Hao Oolong, White-Tipped Oolong, Eastern Beauty and Imperial Oolong) has a uniquely peachy, honeyed flavor due to tiny insects that gnaw at the plant as it grows, causing the plant to change its chemical balance.
Wuyi Oolongs are typically twisted. They have a deep, mineral-like flavor due to their unique terroir. Sometimes, they are heavily roasted for an espresso-like flavor.
Phoenix Oolong teas are generally twisted, and are differentiated by the varietal used to make them. Different Phoenix teas are named for their fragrances, which are determined by the varietal from which they are grown. Many Phoenix Oolong are very rare.
There are also many flavored oolongs available in the U.S. Common oolong flavors include rose, osmanthus, chrysanthemum, mint and citrus.
Some high quality oolongs are said to induce a state called tea drunkenness.