What is evolution and why does it happen?

Evolution is a constant process that allows life forms to adapt to a changing environment.

How does the environment change, though? 

Well, there are several sources of variation in every ecosystem. Variation can be organic, i.e. the arrival of a new species that may compete for food or territory with the pre-existing species in the ecosystem, or inorganic, such as a change in soil composition, or even climate change.

These changes are generally slow, and so one would be tempted to think that most organisms are not evolving constantly... 

And that's where remembering that the organic parts of the ecosystem are also part of an organism's environment: take hares and wolves as an example. 

Hares run fast, which allows them to avoid predation from wolves. But wolves need to eat hares, or else they will die. So wolves are under pressure from hares to run faster. If wolves run faster, hares will get caught and die. So hares are under pressure from wolves to run even faster. 

As a result of environmental changes in a predator species, a prey species evolves, and vice versa. 

Evolution is the constant change of organisms in response to changes in their environment.