Rizwan: A Tin whistle (aka Irish flute) is essentially just a length of metal tubing with six holes of varying sizes drilled in it at specified spacings, and a Fipple mouthpiece. read more
Mainly, the difference is that a tin whistle has a fipple, and the flute does not. Strictly speaking, a tin whistle is a kind of flute called a duct flute, owing to it having a fipple. read more