Most of the people voted at yahoo answers that Haku is a boy who dresses like a girl.
One of them was also saying:
"Haku is a boy but had me fooled until Haku seen Naruto checking him out and he told Naruto he is a boy. Haku can easily pass for a girl though. Naruto was trying to concentrate his Chakra to his feet so he can climb a tree and while doing that, Haku was also in the forest so that is how they had the conversation. Kakashi told Naruto, Sakura, and Sasuke that learning to climb up a tree while concentrating their Chakra in their feet would help them in their training to defeat Zabuza."

Haku was born in a snowy village in the Land of Water. During Haku's childhood, wars ravaged the Land of Water, and since those with kekkei genkai were commonly used as the most terrifying tools of war, many people irrationally dreaded and hated these people.Haku's mother possessed a Kekkei Genkai but kept it a secret from her husband and the rest of the village. Her ability was that of controlling water. Haku awakened this power when he was very young. Upon revealing this to his mother, she scolded him and ordered him to keep it a secret. When Haku's father found out that they were both possessors of the dreaded Kekkei Genkai, he murdered his wife and attempted to do the same to Haku. Even though he was still very young, Haku was able to overpower and kill him, apparently without realizing it until after the deed was already over. He wandered the Wave Country as an unwanted, beggar child until he was found by Zabuza, who agreed to take him in and wanted to merely use him as a tool. Haku is fiercely protective of and devoted to the older ninja to an extreme, accepting his life as essentially Zabuza's possession and asking Naruto to kill him when he feared he could no longer be useful to his master.