Last week a fallen space rock caused all sorts of chaos in Chelyabinsk, Russia. Its supersonic shock wave smashed buildings, injured about 1,000 citizens, and caused a boom in the trade of extra-terrestrial rock samples. A big reason it was so dramatic was the sheer size and mass of the rock— about 55 feet and 10,000 tons.
But Chelyabinsk's not alone. Every day tons of material rains down on Earth's atmosphere and some of it hits the ground. In Siberia there's a large enough population that can spot meteor events and find the remains. This obviously isn't the case for the Pacific ocean, which is hit by debris just as often.
Turns out, there's simply a lot of solid matter up there roaming the blackness near our Sun. Some of this stuff is rock, some of it is iron, some of it is made of rarer metals...and all of it is left over from the time when our planet was being formed, back when the Sun was a newborn star. And sometimes the path a lonely lump is following—one it has been on for billions of years—intersects with where our fragile little planet happens to be.
When it hits the atmosphere at terrific speeds (and I do mean terrific—think muuuuch faster than the speed of sound) friction heats the falling object up and nearly always burns it into a mere puff of ions. Most "shooting stars" you've seen are actually the result of a tiny lump of space rock whacking into the atmosphere like this. This, technically, is considered a meteor.
But bigger meteors can survive the fiery trip and do hit the ground. These are meteorites. Either the impact happens in the present and we seek out and find the debris, or it happened hundreds or thousands of years ago and debris is found much later when someone digs up a field to find something they weren't expecting.
Don't panic! Impacts are pretty rare. In the slides above, check out a bunch of the hot rocks that have, like David Bowie, fallen to Earth.