Common uses of the terms:

shounen-ai  term used when a story has homosexual overtones or themes between two of the male characters.  Usually the main character(s) is involved and sexual advances, insinuations, and scenes are curbed

yaoi  term used when a story has homosexual overtones or themes between two of the male characters and the characters share a psyhical relationship. Usually the main character(s) is involved and sexual advances, insinuations, and scenes are present.

Lemon term used to denote stories with sexually interaction between characters.

Slash fan fiction depicting a homosexual relationship between two characters, term is used with series not originating from Japan

Actual meanings:

shounen-ai  term used when a story has homosexual overtones or themes between two of the male characters.  Usually the main character(s) is involved.  All characters are original creation.

yaoi  term used when a story has homosexual overtones or themes between two of the male characters and the characters share a psyhical relationship. Using Characters created by someone else.

"Yaoi" is a Japanese acronym for "Yama-nashi, Ochi-nashi, Imi-nashi", which means "no climax, no meaning, no point". It does not, contrary to some popular opinion, mean "yamete, oshiri itai" (stop it, my ass hurts!).

YAOI- The basic idea of Yaoi is boy/boy or male/male sexual relations. Flat out, it's Gay, literally. So if you can't dig it or handle it or have to closed up of a mind, stay away.

  LEMON- The basic term is used to describe the level of 'sex' or 'graphic depiction' in a piece of art or work or Fiction. Lemon rates from level R to XXX Which can include oral, anal, rape, torture, bondage and hell any kind of sexual relations depicted Yaoi art.

  LEMON YAOI- Covers all the mediums... Basically it's kinky gay sex art, ranging from boys to men.

BISHONEN- Really just means 'pretty boy'... the term 'bishie' is a slang.