The tilt of the earth on its axis cause's what?

The Earth has seasons because it's rotation axis is not "straight up and down" to the plane of it's orbit around the sun. If the axis were straight up and down, then there would be no changing seasons. Also, days and nights would always be the same length, all year round and everywhere on Earth. The sun would be directly overhead at Noon, every day of the year on the equator, and never anywhere else.