Yuzu is originally a Chinese citrus that has been introduced to Japan during the Nara period (538-710 AD).
Yuzu comes from the old Chinese name of this citrus (yau) and is theorized to have become yu, then yuzu since the juices were used as a vinegar in the early centuries (zuis the suffix form of the word suor vinegar; please refer to: What is Zushi?)
It has a very bumpy skin compared to other citrus and is both used in the unripe (beginning August) and ripe (beginning mid October) stage.
Ripe fruit peels are yellow and are dried and added to make shichimi spice. They are also cut small and placed in soups to give a hint of citrus aroma. The juices have no bitterness, and they are used widely to make sauces, and beverages.
Besides culinary use, the yuzu oils are used for perfume around the world and very popular in adding fragrance to baths in Japan. Whole fruits are often floated in hot baths, and extracts are added to bath crystals as well.