Slash

Slash refers to romance and/or sex between two characters of the same gender. While it can be very good when it's well written, this is rarely case. Often, at least one of the characters is some kind of Mary Sue, or otherwise very OOC. Bad slash is dealt with by the Department of Bad Slash.

Yaoi
Yaoi is a part of slash. It originates from anime and manga. It is nominally targeted towards girls.

Yaoi is more feminine than regular slash, and has two roles: Uke and Seme.

Nearly 9/10 times in a format that is not conventially the home of yaoi, fanon will develop to work the characters that are given the Uke and Seme roles the characteristics that make up these roles.

It is also a given in Lord of the Rings fanfiction that the elf is going to end up the uke much more often than not.