How hazardous are meteors, comets & asteroids?
The collisions of these objects with Earth are basically random events, but still we have some idea how often they happen.
Localized destruction happens every couple of hundreds of years and is somewhat equivalent to a hydrogen bomb. Last such event happened in 1908 near Tunguska river in Siberia. The number of casualties depends on the place of impact (the objects of this size usually explode in the air before reaching the ground, just like an atom bomb). If a city is struck, casualties could be close to a million, while Tunguska event had zero to one reported casualty (reports vary). An impact in the ocean would create a tsunami and definitely produce significant destruction on the nearby seaside. These events usually do not leave a crater and typically involve a 100-meter asteroid or comet.
A smaller object (around 20m diameter) struck Chelyabinsk Oblast in Russia and did cause over 1000 injuries. Most injuries occured when the blast destroyed windows and struck onlookers inside buildings who were looking at the fireball. Fortunately, there are no reported deaths from the Chelyabinsk impact.
A regional destruction happens at intervals on the order of 100,000 years, and devastates an area a size of a mid-sized country. One such event we know of is an impact that occurred 700,000 years ago in Southeast Asia. These events usually involve 1 km sized asteroids an leave a craters tens of kilometers across.
A global destruction happens less often than every 10 million years and involves an impact of 10 km asteroid making a 100+ km crater. K-T event, which caused the extinction of dinosaurs and other contemporary creatures falls into this category. The amount of destruction depends on the properties of rock in which the crater is being excavated. Unless acidic chemicals are released into the atmosphere such an impact does not necessarily have to produce a mass extinction. In any case, such an impact today would cause casualties among humans in the billions.
It is highly unlikely that a regional or global destruction would occur anytime soon (next couple of centuries) since we have already discovered most of near Earth asteroids larger than 1 km, and none of them seem to be heading this way. A localized impact has a less than a percent chance to happen in any given year, so the level of risk at any given place or time is also low.
Concerning smaller meteorites that hit the ground, they are a very low hazard and no human was ever reported being killed by a small meteorite (while one person was missing after Tunguska). I heard a story that a dog was killed by a meteorite that fell in 1911 in Nakhla, Egypt, and there were also instances of material damage. Still, traffic, pollution and even lightnings are much more dangerous than small meteorites.
Updated by Everett Schlawin on July 18, 2015.