Femmeslash (also known as femslash, girlslash, yuri, or shoujo-ai) is a variation on the traditional definition of slash -- homosexual/homoerotic/homoromantic fanfiction. Femmeslash is, essentially, putting female characters together in a romantic and/or sexual relationship.
Femmeslash is a wide and varied genre, ranging from epic romances to hardcore pornography to lighthearted romps to darkest horror. Also varied is its audience—while it may seem that the ideal audience for femmeslash is a purely heterosexual male audience (as most female/female pornography is geared towards the male viewer), femmeslash has a very strong and outspoken female audience.
Typical femmeslash sites
FemFic.org _ A general femslash directory (http://www.femfic.org/)
GirlBitz _ Archive of femslash fiction and video clips (http://girlbitz.com/)
Altfic is a rather broad designation covering traditional literary genres from action/adventure to erotica to romance to pornography to comedy.
Altfic's version of Xena and Gabrielle, or their respective Ubers, are less polarized in terms of their gender traits, so their relative masculinity or femininity is usually unchanged by the story's conclusion.
This is an unspoken premise of almost all Altfic, romantic or not, and it is the significant differentiator from traditional heterosexual romance.