Definitions From Japan: BL, Yaoi, June
The following are an attempt to define terms as they are currently used in Japan, with some historical information to give you a feel for their evolution and a few examples where examples might be useful. Some of these terms are subject to changes in fads and as this is a rapidly changing publishing field that is continuing to evolve, terms will evolve as well.
Boys' love (BL) is the common term used by the publishing industry to categorize works focusing on male/male relationships marketed at women. Historically these works were referred to as June, but most commercial works are now called BL. The change in terminology was probably due to the negative connotations of the term yaoi and the association with a specific publication of the term June.
However, it does not include gay publications.
Boys' love is not referred to as shounen-ai. Boys' love and shounen-ai are two different terms. Boys' love is also referred to as BL, boys', boys-mono. Punctuation and capitalization vary, so you will see boys love, boys' love, boy's love, bl, BL. We have chosen to standardize on boys' love and BL.
While early fans of BL were probably fans of doujinshi, most Japanese fans on the net these days appear to be fans of commercial BL work - there are over a hundred such works published every month, more than two-thirds of them novels and the rest manga. BL novels are immensely popular - the number of novels published each month outnumbers manga tankoubon by about 2 to 1.
Some Westerners object to the definition of the category as being written for women. The target market is determined by advertising and is primarily women, although there are publications, such as Zettai Reido, which have multiple target audiences. That does not mean that men don't read BL, merely that the audience the advertisers, editors, and authors have in mind is women.
Note that BL can also be used as a content descriptor/trope term. When used this way, BL works cross multiple marketing categories.
June was an early publication featuring male/male stories in the tanbi style. People used to refer to the category of male/male relationships targeted at the female audience as June, but since that was a trade name for a magazine, that meaning of the term has fallen into disuse. The category has evolved and changed so much and the types of stories so varied that the entire category is now called BL by the industry and most fans. In some places, including Comiket, original stories are still called 'sousaku (original) June'.
Lady's comics are comics written for older women, probably from age 20 up. It parallels men's manga in age range and range of story. Lady's stories are usually about characters assumed to be heterosexual, but can be about or include male/male relations. Many Lady's and comics contain homosexual characters and homoerotic/homosexual themes, including sex scenes, but they're not considered BL (mainly a commercial categorization) or yaoi (main theme of the story is sex). Lady's and shoujo manga that include male/male elements or homoerotic overtones are sometimes referred to as having 'homo relationship' or 'homo/gay character' and include New York, New York by Ragawa Marimo and Ningen Club by Teradate Kasuko.
Shoujo comics are written for girls, from the approximate ages of six to 16. It parallels shounen (boy) manga in age range and range of story. Shoujo stories are usually about characters assumed to be heterosexual, but can be about or include male/male relationships, including sex. But they are still considered shoujo comics. Shoujo manga that include male/male elements or homoerotic overtones include Banana Fish by Yoshida Akimi, and Boys Next Door by Yuki Kaori.
Many Westerners seem to think any 'strong friendship' between two men, such as in Pet Shop of Horrors by Akino Matsuri, is yaoi and necessarily hints at a sexual relationship, but it doesn't. What some Westerners choose to call shounen-ai (works with no sex scenes) is a regular part of shoujo and Lady's categories. Hints appear all over the place as well as characters that are downright homosexual. It means nothing by Japanese standards. It's just shoujo with homoerotic hints. Japanese fans are so used to this, most of them don't even think of them as being unusual or homoerotic. Girls who are not BL fans are likely to be disgusted by outright BL even if they're not bothered at all by homoerotic hints in shoujo.
Shounen-ai is an obsolete term. Shounen-ai refers to stories about strong relationships between pubescent or pre-pubescent boys. The stories featured angsty, poetic, platonic or romantic relationships. It is used only to point to shoujo manga written in the 70's and early 80's by authors such as Hagio Moto, Takemiya Keiko, etc. (Titles such as Gymnasium in November, Heart of Thomas, and Song of Wind and Trees.) Shounen-ai is no longer written, ceased to exist as a sub-category of shoujo long ago, and this term has long since fallen into disuse. Later stories with male/male sexual relationships are termed yaoi, tanbi, june, or boys' love.
Shounen-ai is not the same term as boys' love.
The current common usage of shounen-ai is to refer to adults who like young boys (i.e. pedophiles).
Tanbi is no longer written. Tanbi is a word meaning 'the worship and pursuit of beauty'. It was used to describe the early male/male stories that mainly ran in June. June was heavily influenced by a well-known author and literary critic who used multiple pen names, so many of the stories utilized a high literary style. These stories came to be called tanbi - stories written for beauty and pursuit of beauty only. Tanbi style includes flowery language and uncommon kanji/words, which makes it a difficult read for foreigners.
The tanbi style is mainly a thing of the past. It has been replaced by BL stories - mass written, easy-to-read stories. Even authors known for their tanbi works like Yoshihara Rieko (Ai no Kusabi) now write mainstream BL and no longer use tanbi style. It's probably because tanbi, like its name, pursues beauty both in language and storyline. It's not simple and fast-paced like the modern BL stories.
Tanbi is like shounen-ai, no longer written but important in the evolution of modern BL.
The term yaoi was originally used to point to badly drawn doujinshi. It later came to be used to point to doujinshi with male/male sex scenes. It now can also be used to refer to sex scenes in any BL manga, or indicate that such scenes exist in a work, or to refer to commercial works that consist mostly of such scenes. For example, Zettai Reido, Boy's Pierce, and Comic June are referred to as BL or yaoi interchangeably.
For a while, the word June was used for original male/male stories and manga, but since June is a trade name for a commercial publication, it has been largely replaced with BL. Commercially published works that many Westerners call yaoi fall into the category of BL in Japan.
Many Westerners use yaoi as an umbrella term to mean any story that includes any male/male relationship and is linked to Japan - commercial manga, anime, games, game-based slash fiction, English fan fiction, fan art, etc. just as BL is used as an umbrella term in Japan. In Japan the term yaoi is limited to doujinshi and sex scenes, because of its negative connotations. Referring to a commercial work as yaoi may be considered offensive. Referring to a commercial BL, Lady's, or shoujo mangaka as a yaoi mangaka may also be offensive. Many titles which are shoujo, such as Yami no Matsuei by Matsushita Youko and Bronze by Ozaki Minami, are categorized by Westerners as yaoi.