What are comets made of? Good question! Comet nuclei are loose collections of ice, dust and small rocky particles, ranging from a few kilometers to tens of kilometers across.
As a comet approaches the inner solar system, solar radiation causes the volatile materials within it to vaporize and stream out of the nucleus, carrying dust away with them.
The streams of dust and gas form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the radiation pressure of the Sun and solar wind cause a tail to form. The tail always points away from the sun. See, down, what interplanetary dust particles lurk inside a comet (infograph).
Infograph Source: Jpl.nasa.gov ; Text Source: http://goo.gl/7mxt5V