Now we know otherwise. Every year, like clockwork, the earth passes through the debris trail of periodic comet Swift-Tuttle between July 23 and August 22. The comet is almost never in sight, but previous orbits have left tiny bits behind in streams woven together like a shredded ribbon. When the earth passes through one of those streams the tiny particles streak though our atmosphere and "ablate" -- they vaporize and burn up rapidly leaving bright trails through the night sky. Occasionally a bigger bit will sizzle and pop while it streaks across a larger segment of sky, and once in a great while one will burn from horizon to horizon -- certainly not every year, but there was one last year.
Scientists now have the equipment to accurately predict when we will pass through one of the strands of the ribbon, and this year, we will pass through a thick one on August 12. As usual, the best viewing time will be between midnight and dawn -- just because that's when it's darkest and that's when the particles will be coming straight at the part of the planet you happen to be sitting on. Best viewing will, of course, be away from city lights, but even in big cities, like here in Rome, the brighter ones should be visible. The moon will be up, so you should watch from the shady side of a building -- away from the moon and blocking local lights as much as possible. Lay down flat on your back with your feet pointed south and just look straight up.
As the name implies, the Perseids radiate from the constellation Perseus, but, even if you know where that is, it's not best to look toward the direction they are coming from: meteors directly in front of you will not move much and fainter ones might be missed -- all you'd see would be dots that glow and quickly disappear. (If you simply must find Perseus, look North-East and about 40 degrees above the horizon between midnight and 1 AM on August 12.)
Where and what they are: http://comets.amsmeteors.org/meteors/showers/perseids.html
How they were "discovered": http://www.skypub.com/sights/meteors/perseids/discovery.html
Perseids for kids (but lots of info
adults can use too): http://www.skypub.com/sights/meteors/perseids/discovery.html
A huge list of links to everything you ever wanted to know about meteors: http://wwwvms.utexas.edu/~ecannon/meteorlinks.html