Its literal meaning in Japanese is 'boy love'. In Japan, it's associated with paedophilia and is an obsolete term. In the West, however, it's taken on new meaning to refer to the less graphic depictions of male/male romantic relationships aimed at women. More descriptive art and fiction are often referred to as yaoi. Female/female relationships are called either shoujoai or yuri, depending on how graphic they are.
Many people have different attitudes to its use. Others will demand you use the term BL/boy's love as they do in Japan, some will say it covers all m/m relationships, but most follow the above description.
"This story is shounen ai, so please don't read if that offends you!"
Retro term once used in Japan to depics softcore romantic relationships between men, aimed for women's entertainment (& not for mockery either). Some people who have touble adjusting or reading Japanese or staying updated still call it Shounenai, but now it's called BL.
Go to Google.om, switch settings to Japanese language only, type in BL. You'll gets tons & tons of links. For you buffs who know katakana & cut-&-paste, try adding wods like gallery & illust for picture BL sites.