Slash refers to romance and/or sex between two characters of the same gender. Like all kinds of fanfic, it can be very good when it's well written, but very often it is not. 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.
Slash gets its name from the way writers denote romantic pairings when advising their readers of them in the story's summary: Person A/Person B. Note the slash ( / ) character.
Slash is known by many code names; in fact, 'slash' is probably the most covert term used (at the very least, a non-ficcer would be hard-pressed to understand what 'slash' refers to based on the term alone). However, terms such as 'mansex', 'mansexing', 'gaysex', 'buttsex', 'buttsecks', 'teh gaysecks', and other derivatives are also common, and not very subtle.
Femslash refers specifically to slash between two female characters.
Slash Conventions Edit
There are several different ways slash stories can be set up.
First, and least imaginative, is when the author assumes that the relationship is longstanding and thus not needful of justification. This approach effectively cuts out any plot tension or conflict from the potentially fruitful source of family and friends, who, depending on continuum, may or may not be particularly happy with their relation/friend being in a homosexual relationship.
Another popular scenario is when mutual attraction is discovered accidentally through a set of circumstances including but not limited to: drunkenness, being in a fight (either on the same side or opposing), being forced into an arranged relationship due to hitherto unheard-of social customs, being imprisoned together, sex-pollen or aphrodisiac fruit, etc.
Sometimes a slash story can be non-consensual ('non-con'), in which one character forces another to engage in actions against their will. Rather improbably, non-con stories occasionally end in the forced party realising they were in love all along.
Generally, the most successful set-up for a slash story is one in which the relationship is shown developing and the consequences of actions are explored; i.e., the requirements are the same as for most other kinds of fiction.
Yaoi has slightly different conventions from 'regular' slash, including casting the two main characters into submissive/dominant roles: uke and seme. Nearly nine times out of ten, in a canon that is not conventionally the home of yaoi, fanon will tend to push one character or another into one of these roles.