If you'll excuse the analogy, the immediate surroundings in space are kind of like Brooklyn. It's becoming increasingly crowded, expensive, and ultimately full of things that shouldn't be there in the first place. Unlike Brooklyn, however, space is full of space debris: small amounts of (basically) trash that humans have gotten in our orbit. The ISS has cataloged about 500,000 of these small pieces and they hurtle around our planet at about 15,000mph. Or 14.17 g-force. Or 24,140kph. 

An anonymous user on Reddit, who apparently works in the aerospace field, posted an image of what a 1/2oz of space debris can do to a block of solid aluminum. This test was done by a light-gas gun in close quarters and shows how much damage even a tiny amount of space debris can do: 

Pretty scary, huh? It should be noted that although this looks enormous, the crater is about 5 inches deep. Having said that, it's caused by something about the size and weight of an eraser on the end of a pencil. 

The ISS (International Space Station) is about the size of a football field, and thus an easy target for space debris. To solve this, it has to move their orbit to make sure they don't get hit. Every once in a while, they get hit by pieces the size of a paint chip and need to repair the ship for weeks. 

What's that? Do you want another interesting space debris fact? Well, during the height of the Cold War in 1965-1967, the U.S. fired hundreds of thousands of tiny needle into space to try for what could best be described as high-powered radio signals. This, however, didn't work nearly as well as planned and the needles just clumped together into groups — turning these needles into high-velocity projectiles. 50 years later, there are about 38 of these clumps still in orbit, although sometimes they enter the Earth's atmosphere and burn up.