If you are lucky enough, you might have seen a tailed star moving in the sky very fast. These are comets. Comets are celestial bodies which revolve around the Sun. 

Sometimes when they come too close to Earth, due to the earth’s gravity, it ends up falling on the earth.

Outside the orbit of Neptune, there is a very large cluster of comets called the Oort cloud. Occasionally any-body from here gets away from this cloud and starts revolving around the Sun in an elliptical orbit. While orbiting, they at times in the gravitation field of the Earth. 

A comet need not always fall while passing by the Earth. It may simply come closer to Earth and then bypass it at high speeds.

Why do Comets have Tails..!  When a comet enters the gravitational field of earth, due to dense air, it becomes very hot due to drag. The Gaseous elements in the comet start burning and due to less mass (gaseous phase) look like a tail (just like from a rocket).