Yaoi is a genre of publishing in Japan, including yaoi manga, doujinshi, anime and other artwork. It focus on relationship between 2 male characters and has detailed sexual description.
Yaoi is not equivalent to Shounen Ai or Boys’ love. The latter have no direct sexual description.
Strictly speaking, we should pronounce three vowels(ya-o-i) each instead together. It’s wrong to pronounce like /jaʊi/ or /jeɪɔɪ/.
Yaoi’s etymology is very interesting. It comes from Japanese phrase 「ヤマなし、オチなし、意味なし」 (yama nashi, ochi nashi, imi nashi) for short, which means: No peak, no point, no problem. And also it’s dubbed “Yamete! Oshiri ga itai!” which means “Stop it! My butt is hurting.” (XD)
Girls fond of Yaoi are called Fujoshi. Some people may think they’re weird, but actually, they are just so kind to the special group of Gay in this cruel society. And they’re so imaginative that romantic stories are just flashing into their mind when they see two boys together.
In Japan, Kadokawa Shoten Publishing Co., Ltd publishes good yaoi manga, such as Junjou Romantica, Sekaiichi Hatsukoi.