THE WORD: Moshi moshi
THE MEANING: How you say "hello" on the phone. Don't say "konichiwa!" You can also buy this rad shirt here.
THE WORD: bento
THE MEANING: a packed meal held (usually) in one box. It can look simple, or it can look like a Hello Kitty cartoon.
THE WORD: purikura
THE MEANING: a photo-booth, often with stickers--and often with crazy options that will turn you into an anime character.
THE WORD: karaoke
THE MEANING: you probably think you know it, but you will live it when you're in Japan. Seriously, the scene in Lost in Translation nailed it.
THE WORD: gyaru
THE MEANING: the Japan-ization of the word "gal," you'll see them way more than you'll see, say, goth Lolitas. They are super girly-girls.