Asteroid impacts that have the capacity to cause an extinction level event are few and far between. However, they do happen and the geological record has found many examples of large body impacts with the Earth.
Most impacts, however, cause local damage and are not considered extinction level events. The earth is hit many times and accumulates about 100 tons of matter every day. The larger objects hit with less frequency and the really large objects that can inflict huge damage are even less frequent.
The odds are that school-bus sized asteroid may hit every thousand years; a larger asteroid in the region of 500 feet may hit every fifty thousand years and the largest every billion years or so. The or so is important - no one can predict with accuracy when an object will collide with the Earth. The last extinction level asteroid wiped out the dinosaurs only 65 million years ago so we may not be due another one for a long time or it could be tomorrow.
An asteroid does not need to be super-massive to cause an extinction level event. The dinosaur killer is believed to have been about 5-10 miles across - tiny compared to the size of the planet.
The damage is caused by the output of the impact - an asteroid that size has the impact of several million nuclear weapons and can eject large amounts of dust into the atmosphere. If the impact is powerful enough, this would be sufficient to cause a "nuclear winter"; global temperatures would drop significantly and the sky would remain dark for at least several years. The outcome is that population would quickly run out of food an resources and there would be an extinction in that large numbers of species, including humans, would be taken to the brink of extinction.