Not long ago, many people thought that comets were a sign that something bad was about to happen to them. People didn't understand how objects in the sky moved, so the sight of a comet must have been very disturbing. There are many historical records and works of art which record the appearance of comets and link them with terrible events such as wars or plagues.
Now we know that comets are lumps of ice and dust that periodically come into the center of the solar system from somewhere in its outer reaches, and that some comets make repeated trips. When comets get close enough to the Sun, heat makes them start to evaporate. Jets of gas and dust form long tails that we can see from Earth. These tails can sometimes be millions of miles long.