Why do we have seasons, and what are they?

As the earth spins on its axis, producing night and day, it also moves about the sun in an elliptical (elongated circle) orbit that requires about 365 1/4 days to complete. The earth's spin axis is tilted with respect to its orbital plane. This is what causes the seasons.

There are four primary seasons: Spring, Summer, Autumn & Winter. The start dates of the seasons themselves are different depending on what hemisphere you're located in. Spring & Summer are the warmer seasons. Autumn is typically mild, and Winter is the coldest season of the year.