Extinction occurs when there are no individuals of an entire species alive.
Extinction can occur when organisms have failed to adapt to a changing environment. Animals that have adapted less may fail to reproduce and could eventually die off leaving the species extinct. This plays a role in evolution as some species die off and some will then continue to live and evolve.
There are a lot of factors that can have an effect on extinction:
A change in the environment (climate change)
A new, un-treatable disease
A new predator
An event such as a meteor colliding with the Earth such as the that wiped out the dinosaurs
Many species may become extinct through human activities. Since humans are very successful predators, they are very good at hunting for food, competing fro space and gathering materials. all of which can lead to the death of many animals. Deforestation is an example where millions of organisms loose their homes and food due to loss of habitats, This can ultimately lead to extinction due to a human activity.