"Yatta!", which has become one of the most famous internet memes next to Hatten är din and All your Base are belong to us was origninally created by a japanese comedy duo called Ucchan-Nanchan (their real names are Teruyoshi Uchimura and Kiyotaka Nanbara), who have a very successful show on TV. On one of their tv shows ( they actually have multiple shows on various stations, such as NTV, Asahi TV and TBS) they started "Yatta!" as a sketch on that show, in a way similar to the early apprearances of the Blues Brothers on SNL. The sketch was about a fictional music group called Happa-tai (which could be translated as Leaf Squad, Leaf Group or Leaf Club). The members of this group wear only a leaf, singing their spirited ode to happyness, Yatta! (which, as mentioned above, is an exclamation of satisfaction, or happyness, similar to the english Yeah or Yippee!). The group consists of Ucchan and Nanchan and a couple of the regular guests. The sketch was positively received by the audience, and was thus refined, reshaped, and appeared various times in various forms on the show, until the song was finally released on CD. The video circulating on the internet is probably the official music video for the song.
As the original japanese text (absolutely relevant for late-night nomikai-karaoke sessions) has not been noded yet, I'll add it below. The parts in brackets are sung by everybody...
G R Double-E N Leaves
G R Double-E N Leaves
It's so easy! Happy go lucky!
We are the world! We did it!
Hyuu! Hyuu! Hyuu! Hyuu! Osu! Osu! Osu! Osu! (Ai!)
Happa ichimai areba ii. Ikite iru kara lucky da!
Yannaru kurai kenkoh da.
Everybody say yatta!
Nippon kyuukyuu (demo)
Ashita wa wonderful
Ijiwaru saretemo futon haireba
Guu guu guu guu! Pass pass pass pass (Ohayo--!)
Neoki de jump
Donna ii koto aru daroh ikite ita kara lucky da!
Kimi ga kawareba
Sekai mo kawaru
Marugoshi dakara saikoh da massugu tattara kimochi ii--!
O-mizu nondara umee! (Yatta!)
Hi ni atattara attakee! (Yatta!)
Hara kara warattara omoshiree! (Yatta! Yatta!)
Inu-katte mitara kawaii! (Yatta!)
Surechigai-zama hohoemi kureta
Nido to aenaku-tatte ii kimi ga ita kara lucky da!
Heisei fukyoh seiji fushin
Reset sae surya saikoh da! Minna iru kara tanoshii--!
Happa ichimai areba ii. Minna issho da happy da!
Iki wo sueru
Iki wo hakeru
Yannaru gurai kenkoh da! Everybody say yatta!
2001, Happa-Tai, released by Pony Canyon.