Wulong/Oolong tea is a partially oxidized tea, and the level of oxidation can vary widely from around 5% to 100%. Taiwanese Wulongs are traditionally known for having minimal oxidation whereas Chinese Wulongs are known for having high levels of oxidation. Wulongs can also be rolled very tightly into balls or dried as full leaves. Wulong is one of the more complex categories of tea; the processing methods can be extremely labor intensive.
Wulong and Oolong are two different ways to spell the same two characters in Chinese: 烏龍 People typically use Wulong if the tea is from China and Oolong if the tea is from Taiwan. Both are acceptable transliterations. The direct translation of Wu Long is "Black Dragon".