Actually, billions of meteoroids hit the Earth's atmosphere every day, but only tiny fraction of those are large enough to be seen, much less to actually land on the surface before completely burning up. Meteorites may be divided by discovery into two categories. Those rare meteorites which are seen to strike the ground are called falls. The rest, which are stumbled upon and noticed because they appear to be unusual looking rocks, are called finds. Although most meteorites are stony, irons are most commonly found because they more obviously differ from Earthly rocks.
Irons are very dense, and magnetic, and appear to have been somewhat melted by their fiery fall.
At a number of specific times during the year, the number of meteors observed increases greatly, in some cases, exceeding 60 meteors per hour. These are called meteor showers. The material that produces them is the left-behind debris of comets, which release gas, dust, and rocks as they melt on their approach to the heat of the Sun. The comets themselves may or may not still exist, but the material they leave behind still does, and it follows the path of the comet's orbit. Each year as the Earth crosses that comet's path, it collides with the cometary debris, and we see the meteor shower.