Tomodachi is the Japanse word for friend. Tomodachi is written with two kanji: 友 (tomo, friend)
and 達 (-tachi, attain). The first kanji comes from the Chinese you and represents two hands (又 right and 左 left) working together. The second kanji comes from the Chinese da and isn't relevant to the etymology (it's phonetic, "a word about moving that sounds like da" = attain). The Japanse word itself then stems from the idea that working together to accomplish a task creates friends. For the Japanese this will generally be true, as the members of your ka sei (課制, company work group) are often the people you socialize with the most. A familiar abbreviation of the term, just tomo, translates closer to "buddy" or "pal" (私の友).
I used unicode html entities for the kanji. Here's why.