Comets are dirty snowballs -- loose accumulations of ice and dust formed at the very beginning of the solar system. Comets are on very elliptical orbits -- they spend most of their time in the deep freeze of the outer solar system, but every so often (from a few to many thousands of years), they approach the sun. The sun melts some of the ice, which forms the head and tail of the comet which we see on earth.
This year marks the return of comet Schwassmann-Wachmann 3, a comet that orbits the sun once every five-and-a-half years. Eleven years ago, observers from Earth noticed that the comet looked strange -- it was breaking up into multiple pieces. Why? We can't be certain, but comets are pretty loosely held together. It could be that the repeated freezing and thawing of the comet finally broke it apart. This phenomenon is familiar to residents of colder climates, who can watch their streets break up into pot-hole ridden throughways every spring.
This May, the pieces of Schwassmann-Wachmann 3 will fly near to the Earth. If you are able to go to a dark field, you may be able to see a string of faint pearls in the night sky!