What composes the head and the tails of a comet?

Occasionally a comet leaves the Oort Cloud and travels into the inner Solar System toward the Sun. Close to the Sun, the comet grows in size and brightness. The Sun's heat turns the snow on the surface of the snowball nucleus to gas, and some dust is released. The gas and dust form a bright head around the nucleus , the coma. More gas and dust is blown away and forms two tails. The dust tail is yellowish-white, the gas tail is bluish. The tail always points away from the Sun because light and other forms of radiation from the Sun push against the minute particles that are present within the tail.