The earth orbits the sun, completing one orbit every (approx) 365.25 days. But whilst it spins on its North-South axis (causing day and night) it also tilts slightly, causing the North Pole to 'lean'...
winter, spring, summer and autumn
The Earth reaches four points on it's journey around the sun, the poles both experience four seasons, however some are longer than others.
summer winter spring fall
Because of how the earth rotates aroumd the sun.