It's a Maori dance that the All Blacks, the New Zealand Rugby Team, perform before every match. Kinda think Martial Arts dancing, minus the weapons and with more chanting.
There's some controversy currently in the Rugby world because evidently the Scotish fans think the last move in the new Haka (a thumb slice across the throat) is too vulgar.
And -they're- the one's playing Bagpipes before the matches. >D

Check this link for more information:
http://www.haka.co.nz/haka.php