Visitors from the outer reaches - Comets
My best description of what a comet is, is a very dirty and large snowball. A comet is made up of frozen gasses, water ice, dust, and maybe small chunks of rock or gravel. As to how a comet forms can only be speculated on, as the formation of our Solar System is just a theory and has changed often. The strongest theory is that that comets are what was left over when our Solar System was made.
Comets spend most all their time in the dark and extreame cold well past Pluto where the Sun looks like a extremaly bright star and there is little or no warmth from the Sun reaches. Eventually something disturbs a comets orbit and changes it to one that sends it to the inner Solar System. That something is most probably another comet and it is just as likly to get flung out of the Solar System as into the Solar System.
As a comet nears the Sun it warms up. The various gasses all have different vaporizing points (a lot of things do not melt to a liquid in space). As the various gasses vaporize and stream off they carry with it dust. The Sun is constantly throwing off plasma (Highly ionized matter) and is called the Solar Wind. The Solar Wind catches the comets gasses and dust and blows it away from the Sun. The tail never indicaties the direction the comet is going, only the direction the Solar Wind is blowing and that is always away from the Sun. The tail of a comet can streach for millions and millions of miles.
Sometimes comets get close to a planet and get its orbit changed. Some will be changed so that they never stray to far from the sun. These eventually have all their gasses vaporized off and end up looking no different than an asteroid. Others can get a large boost, by passing near a planet, that throws them out of the Solar System forever.Comets, when they come in to the inner Solar System, are always moving and there is no way I can point you directly to one. It will help to get a copy of Astronomy or Sky & Telescope magazine for the present month to know if a comet is visible an where.