Gay Romance: Similar to typical romance novels, but they feature male on male characters.

Erotica: These stories are usually designed to stimulate arousal. They will often contain many sex scenes throughout the story. (In most cases, the story revolves around sex). 

Yaoi: Japanese style stories that feature male on male relationships. Most of these are written for a female audience. 
     *often contains violence, taboo subject matter, and characters with 'pretty' or feminine features. 
     *stories specifically featuring young men with long hair or pretty features are referred to as 'bishounen'. 
   *Japanese style stories that are usually more focused on men who have expected male features. (Hairy bodies, physical labor jobs). These are referred to as bara:

Boys Love: These are romance stories but often have more sexual angles than a regular romances but less than erotica.  (Some are listed in place of yaoi and are just as sexually intense/explicit. Viewers should tread with caution)

Shounen-ai: Japanese style stories that are usually more focused on young or underage boys and themes. Like sports teams, high school drama and the like. 
   *These often have no sex scenes but rather hand holding, flirting etc.