What is the difference between natural selection and evolution?

Evolution is a gradual change in the inherited traits of a population over many generations. 

Natural selection is a mechanism where the members of a population best suited to their environment have the best chance of surviving to pass on their genes. The natural environment selects for these members over others in the population. 

Over many generations, because more of the best suited members have been reproducing, the population all now have the trait that makes them better suited for the environment. 

This population has now evolved to better suit its environment, through the process of natural selection.