Explanation:
http://en.wikipedia.org/wiki/Yatta
"[i]'Yatta!' is a 2001 parody song by a fictional Japanese band called はっぱ隊 (Happa-tai; literally 'Leaf Squad' in Japanese, though they call themselves 'Green Leaves' in English).[/i]"

The Video:
http://www.youtube.com/watch?v=It0i3t_sQAw
yatta!