The earth orbits the sun on a 23 deg angle. Its orbit, like all the planets in the solar system is relatively flat.
A complete rotation of the earth around the sun takes 365.25 days, 1 year.
The .25 adds up, so every 4 years we add an extra day to February, making this a leap year.
As seen in the animation to the right.
When one side of the earth is facing the sun, its day time, while on the other side its night. As the earth spins on its tilted axis, it take 24 hours to make a full rotation (1 day).
So we know how day and night occur, but why do we have seasons? Extended periods of cold and heat.
Its all to do with the 23 degree tilt of the earth.
As you can see if there was no tilt, the amount of heat and energy from the sun would be equal on both halves of the equator.
When we tilt the earth, one part of the equator is exposed to more of the suns rays (summer) than the other (winter).
The animation bellow shows how the earth orbits the sun on its angle, and how the seasons are created. Notice how the sun is almost always hitting the equator. This is why the equator is usually hot.