The Haka is traditionally a war cry or a challenge which originated from the Māori, an indigenous people in New Zealand. The Haka is characterized by the shouting of a chant coupled with matching hand and feet gestures, along with facial contortions which include showing the whites of the eyes and poking out of the tongue.
The Haka I am most 'familiar' with is the war haka, owing to the rugby team The All Black's performance before every match. Apparently, schools also have their own haka and it is of the war kind. So in essence, the Haka has permeated the New Zealand psyche and so it is identifiable as uniquely New Zealand. There are other forms of haka as discussed by Wikipedia but I have not seen those yet.
We are currently in the Rugby World Cup fever and so I have seen a couple more haka performances on TV. The war Haka is performed to raise the spirits of the participants so that they can bravely face the battle looming before them (as Rugby does indeed look like a battle by itself). Its double purpose is to intimidate the 'enemy'.
I had a chance to watch the Haka performed live by some students prior to a rugby match and it can be quite a spectacle versus watching it on TV.
Here is a video of the pre-game 'ceremonies' before a rugby match between New Zealand and Tonga.
New Zealand has the Haka, Tonga has the Sipi Tau (their war dance). Whoa, I learned something new today.