Anime: Hel-LO? Are you looking for a definition here? :)
Chibi: A style of drawing where the character is dramatically shrunken and drawn with overly large, painfully cute eyes. Also known as 'superdeformed.'
Doujinshi: Fan-drawn manga. In Japan, doujinshi are allowed and even encouraged, as long as the print run does not exceed a certain number. Doujinshi do not require an ISBN number. They are not commonly sold in the U.S., but they are becoming increasingly easier to find with the advent of eBay and Internet-based commerce.
Fuku: A Japanese schoolgirl's uniform. It usually looks like a cross between a sailor's dress and Catholic school uniform.
Gijinka: In Digimon doujinshi and fan art, a gijinka is a Digimon drawn with human features.
Hentai: Literally, pervert. Someone who's into the down-and-dirty sex, or a style of manga and doujinshi portraying steamy or kinky lovin'.
Hiragana: This is the first writing system that is taught
to Japanese children and is commonly used to display simple words,
verb conjugation, and particles of speech. Hiragana is
a smoother style of script and has more loops and curves than
the other character-sets. A lot of the characters look very similar,
and they're difficult for a novice to reproduce correctly.
Kanji: Borrowed Chinese script and a pain in the ass to
deal with. Kanji is a series of ideograms representing
ideas as opposed to sounds. It most commonly expresses place names,
most nouns, and certain names. For example, in doujinshi, Wu Fei's
name is most often written in kanji, because his name is
difficult to translate into katakana. (He's also Chinese in origin,
but that's beside the point.) :)
Katakana: A Japanese script commonly used to symbolize borrowed, foreign words, company names, new words in Japanese, and foreign names. Most doujinshi characters' names are written in katakana. The style is blocky and angular and is, in my opinion, the easiest to pick up on.
Manga: Graphic novels, generally commissioned by the creators of a particular series.
Neko: Literally, cat. Often portrayed in doujinshi as a cat-person/furrie. In Gundam Wing doujinshi, Heero tends to be most commonly drawn in neko style, with pointed ears and a tail. While other characters are also drawn with animal-like features (Duo is often a dog/wolf/fox, Wufei is often a bear, and Quatre is often a bunny), I don't believe that technically qualifies as being a neko.
Romaji: The least common 'alphabet' used in written Japanese communication. Romaji is the Romanized alphabet used in most Western script.
Seme: Generally used to describe male/male relationships (but also applicable to female/female or heterosexual couples), the seme is the dominant partner, or 'top.' Depending on the style, a seme can simply be the 'stronger' of the two, or he can be downright forceful. Traditionally, the seme is either taller or older (or both) than his partner, regardless of personality traits.
Shoujo: An artistic style catering to a young, female audience. Gundam Wing is widely regarded as a shoujo-style anime for its use of hot young men and political intrigue.
Shoujo-ai: Female/female love.
Shounen-ai: Male/male love. In doujinshi and in fan fiction, this term is used to describe a relationship where physical pleasures do not go beyond kissing, touching, holding hands, etc. The style is generally more romantic, delving into aspects of a caring relationship between two males.
Uke: The opposite of a seme in a male/male relationship, or the 'bottom.' Uke are usually shorter or younger (or both) than the other partner and are often drawn as wide-eyed, innocent, blushy, and sometimes feminine.
Yaoi: Male/male love, taken to a physical level. In doujinshi style, Soft Yaoi generally designates that there is implied sex, or shows very non-graphic sex scenes, while Hard Yaoi lets it all hang out. Hard Yaoi also tends to portray more forceful and often hentai sex.
Yuri: Female/female love, taken to a physical level much like Yaoi.