Scientists believe that species change over time to better deal with their environment. We call this process evolution.

Evolution does not happen to any organism by itself--it happens to the entire species across a long period of time (generations). This is how it works. Out of all the organisms in the species, the ones that are better at surviving and reproducing will pass their genes on to the next generation more. We call this natural selection. For example, suppose cheetahs that can run the fastest will survive the longest and have the most children. Then, when the children grow up, they will be faster (on average) than the older generation thanks to their parents. Now cheetahs have evolved to run faster. Here's another example: rough-skinned newts can make poison to threaten their predators, garter snakes. In some cases, only the newts that make the most poison can survive. These newts will have the most children, and when the children grow up, rough-skinned newts will have evolved to be more poisonous.

This is too vague to really answer. Evolution happens due to a great many factors and forces, not all of them because of natural selection (for example, mutation and genetic drift are evolution but not natural selection).