Extinction is where all the members of a specific species die out. There are two different types of extinction, normal extinctions and mass extinctions. Normal extinction is when just one or two species die at one time from the same cause; mass extinction is where most species die from one cause.
Extinction of a species can be caused by a number of factors such as competition for food, lack of food, disease or over predation. Anything that causes members of the species to die of quicker than they can reproduce will eventually cause the species to become extinct. Species become extinct every day, not only in the past. Humans have caused many species to become extinct such as the dodo and the Tasmanian tiger. The Tasmanian tiger was over hunted until there were none left and the dodo was wiped out when humans arrived, on the island where they lived, with dogs, pigs and monkeys, which destroyed their nests and ate the dodos, eventually causing them to become extinct.
The extinction of species does play a large part in natural selection. If an organism is not suited to its environment it is less likely to survive and produce offspring, while the better-suited organisms will survive and produce offspring, thus passing on the characteristics that helped them survive onto their offspring. This causes species to gradually evolve over time. So evolution relies on extinction, if an animal is not suited to its environment it will eventually become extinct, as the organisms cannot survive long enough to produce sufficient offspring.
However, not all extinctions happen one at a time, sometimes something can cause most of the species on earth to become extinct in a short period of (geological) time. When most species are wiped out it is called a mass extinction. An example of a mass extinction is that of the dinosaurs and many other species of that time. There is still a debate on what exactly caused it but the most likely reason is that a meteor from space hit the earth and the shockwave, and energy released by it, was enough to destroy much of the life on earth. This type of extinction does not affect evolution in the same way. In the past when mass extinctions have happened, there are only a few species (relative to the number before) left. Only the species that are left and that are well enough suited to the environment after the mass extinction will be able to survive and pass on their genes to the next generation.