text: The earth is tilted on its axis as it orbits the sun.  In the months where the northern hemisphere is tilted towards the sun it is in summer and the southern hemisphere is tilted away from the sun and is in winter.  The other two seasons are the transitions between summer and winter.  The further north or south you go (closer to the poles) the more extreme the difference in conditions between summer and winter.  At the equator, there is very little difference between the seasons.