Kaitou Literally means "mysterious thief" or "phantom thief", but can also mean just plain "thief".
Kami Literally, "god". Kamizoku refers to a race of gods, and "kami-sama" addresses a specific God.
Kanji Japanese, Korean, or Chinese written characters, where one character represents an entire word or concept.
Katakana Kana for short. One of the three Japanese writing systems. This is also a phonetic alphabet, but it's often used to write names or foreign words. For example, a person would write the text of a letter in Hiragana, and then sign their name in Katakana. It's complicated, but that's life.
Kawaii Very, very cute. Bordering on adorable. Can also mean "sweet".
Kaze "Wind".
Kimi "You".
Kirei "Beautiful"... also, "clean".
Kisama "Crap". (Careful where you use this one, though!)
Kite kite? "Guess what?" Also, "listen up!"
-Ko An affectionate suffix, which I believe is basically the equivalent of "chan"--as far as I can tell, it means "child".
Koi Variously, "heart", "love", "lover", and "carp". ^_^
Koibito Literally, "my loved one". Basically, "darling"; used more towards a boyfriend/girlfriend than to a regular friend.
Kokoro "Mind", "heart", "spirit".
Konban wa/Konnichi wa Both are forms of "Hello", but "koban wa" is more like "good evening" and "konnichi wa" is more like "good afternoon".
-Kun Suffix generally used among guys to denote a friendly relationship; not as affectionate as "-chan" but not as respectful as "-san". Sometimes it's a way to identify a buddy (except when it's used teasingly, like Xellos calling Zelgadis "Zel-kun" in certain fanfics). As Renfield points out, it can also be used to address an equal who is younger, or a subordinate.
Kuroi "Black".
Manga A catch-all term for Japanese comic books.
Mazoku Literally, "evil people" (ma-zoku). In anime and fanfic this term means either the monster or demon race, or an individual monster/demon.
Minna "All" or "everyone".
Mizu "Water". My grandfather says it's specifically the term for cold water, but I haven't heard it defined as such by anyone else.
Moshi Moshi "Hello" (mostly used on the phone).
Mou An expression of annoyance (you can usually draw it out the more irritated you are ^_~).
Namagomi According to Jay-chan, this translates as "Raw garbage"; according to Renfield, it can also mean "LIVE garbage". (icky!) This is a pretty serious insult--so kids, don't toss this one around at home (unless the insultee doesn't speak Japanese).
...nan desu ka? "*Blank* nan desu ka" means "what is *blank*?" Usage: "Kanji nan desu ka?" translates as "What is kanji?" From the way I've seen it used, though, it can also have a connotation similar to "what's that all about?" (Thanks, Libby!)
Nani? "What?"
...Ne? Verbal punctuation. A bit like saying "...right?" or "...yes?" at the end of a sentence.
Neko "Cat". Sometimes combined with a name as a kind of affectionate nickname... or to imply that the person concerned IS a cat. ("Neko-chan" means "kitty" or "kitten".)
Nijikon Somebody (such as yours truly) who likes 2-dimensional characters. (Most anime fans, in my experience, have a "nijikon complex" in one way or another. Can you blame us?)
...no Miko (in a name) A Miko (as Bevin-sama points out) is a Japanese priestess or shrine maiden. Female anime fans, who have a preference for a certain character, will sometimes refer to themselves as "the [Character's Name] no Miko," meaning "Priestess of [Character]." You can also use basically any Japanese term in place of a character's name--one of my personal favourites, "Bara no Miko", means "Rose Priestess". (I have become somebody's Miko, but if you wanna know whose, you'll have to ask me yourself...)