'Yaoi' can be considered an interchangeable term with 'shonen-ai', meaning 'boy's love' or any story whose protagonists are males that have great chemistry, and the possibility of becoming an actual homosexual couple is always foreshadowed.But 'yaoi' stories go beyond that and are more explicit in their content, featuring more 'slash' situations.
Believe it or not, the term comes from Saint Seiya slash fanarts from the 80's! 'Yaoi' referred to the potential pairings between the protagonists of the series, especially with Shun of Andromeda.
And after recalling the infamous classic scene with Hyoga in the House of Libra, well...
Now you know where 'Yaoi' comes from :)
And don't forget Aphrodite of Pisces!
Yaoi situations can always potentially emerge in series where all protagonists are boys or men and they practice activities that are supposedly oh-so-masculine, like being samurais, warriors or ninjas. Just ask Naruto and Sasuke ;)