I couldn't help but click on a thread that has "translations" in its title.
General Vocabulary
Kawaii = cute
Moe = moe
Otaku = fanatic, an insanely devoted fan-geek. One can be an otaku for anything, train-otaku, military-otaku, but used in its most general sense (especially on a site like this) refers to a anime/manga/game otaku. Goes far beyond the concept of "fan".
Ero = erotic, rated X, adult, related to sex
Ecchi = pronunciation of "H", the equivalent word to the English "X", as in X-rated. Rarely used nowadays in Japan compared to the 90s, aside to refer directly to the act of sex --> "Ecchi shiyou?" (Wanna have sex?)
Pantsu = panties
Oppai = breasts
Oshiri = butt
Petanko = flat-chested girl
Loli = lolita, young girl
Loli-con = Lolita Complex, an infatuation with younger girls, perhaps underaged. Also used to refer to the actual person, i.e, "He is a lolicon!"
Shoujo = a young girl, a virginal girl.
Shounen = a younger male, not necessarily as young as "shoujo" implies for girls.
Seinen = adult, adult male. Usually refers to younger adulthood. There are two words with this pronunciation, but different kanji, each with a slightly different meaning.
Shota = younger boy
Yuri = female/female relationship, also used to refer to stories/manga based on this sort of story.
Yaoi = male/male relationship, also used to refer to a style of manga very popular with fujoushi (see below)
Shounen-ai = "young men's love". Basically a synonym for yaoi, perhaps less explicit.
Fujoushi = "rotten women", female otaku, especially those infatuated with yaoi.
No-pan = no panties
Pai-pan = state of shaved, hairless female genitals. "Look at her, she's pai-pan!"
Kyonyuu = huge-breasted
Shitapai = underboob (as in picture angle)
Zettai Ryouiki = "Absolute territory", the area on a girl's legs between a girl's thigh-high/knee stockings and the bottom hem of her miniskirt.
Nakadashi = "come inside", internal ejaculation
Ojii / Ojii-san / -jiisan = Old Man, Geezer (appending -san to any personal, company or profession name is an important sign of respect in Japanese culture)
Obaa / Obaa-san / -baasan = Old Lady, Old Hag
Kami = God, a god (also the word for hair)
Kira = "sound" of sparkling, glittering. Used as an effect in manga panels, etc.
Romaji = Roman letters, the alphabet used in English, Spanish, etc. Used to refer to the transliteration of Japanese into the Roman alphabet.
Kanji = Chinese characters, used in Japanese writing
Hiragana = one of two syllabaries (similar to alphabets) used in Japanese writing.
Katakana = one of two syllabaries (similar to alphabets) used in Japanese writing.
Name Suffixes
La-san = Mr./Mrs./Ms. La (general formality used for people not a close friend)
La-chan = Little La (a term of endearment / affection, used on smaller children or females, informal)
La-kun = Young La (used for younger boys or male friends. Also occaisonally used on female subordinates by Japanese bosses, in which case it implies no sense of masculinity)
La-sama = Master La (very formal)