The annual showers arise from concentrations in dust and debris that are in an orbital path, or stream, that crosses the Earth's orbit. Meteors from a stream will enter the Earth's atmosphere from the same general area. The observed effect from the surface is that imaginary lines, traced backwards along the tragectory of each meteor, will intersect within an 8 degree circle. Meteors tragectories that do not intersect within that area are usualy sporadics but may be from another shower. There are times when several showers are active simultaniously, six are active when the Perseids peak on August 12th.
Showers get their names from the constilations their radiant is located in.
Dust and debris blown off from a comet as it nears the sun forms a cloud of dust near the comet. Each particle follows its own orbital path. Through time and individual perturbations of the orbits of each particle, the cloud of debris spreads out. This begins to form a debris stream which, in whole, widely follows the orbital path of the parent comet. Given enough time the stream will spread throughout the entire orbital path.