Shounen-ai does literally mean "boy love" and that's literally what it is. It is the love shared between two boys/men in an anime or manga. Sometimes it's boy meets boy sometimes it's boy meets what he thinks is a girl but really is another boy, and sometimes its best friends start exploring. The various anime/manga that can be classified as shounen-ai range from bright and happy to rather depressing. Basically it ranges from stuff in comparison to Cardcaptor Sakura to stuff like Neon Genesis Evangelion. Shounen-ai, while its own subgenre, can actually be heavily implied. Shinji Ikari and Kaworu Nagisa have an implied shounen-ai relationship in Eva. It's important to remember that shounen-ai is strictly relationships. Once sex becomes involves (shown) it becomes yaoi or even hentai. Below is a crude list of shounen-ai animes and mangas (implied and explicit are intertwined):
Anime:
Manga:
These are not my personal preferences for anime or manga. I like my women. I swear. What? I do!