The Sun seems to follow a path in the sky and because of this observation many early sky watchers thought that the Earth was the center of the solar system and that the Sun orbited around it. Now we know that the opposite is true. We know that this appears to happen because the Earth rotates. The Earth rotates from west to east so the Sun seems to rise in the east and set in the west. When the Earth turns away from the Sun it seems to set. It grows dark and we experience night time. The sun seems to move across the sky as it rises and sets but in reality we are moving relative to the Sun. Likewise the stars seem to move but in reality they are pretty much stationary relative to each other. We on Earth are the ones in motion.
The seasons occur because the Earth is tilted on its axis. The tilt stays nearly the same and in the same direction as it moves around the sun. Because of this consistent tilt the Sun's rays fall more directly in the Northern part of the Earth for part of the year and directly on the Southern part of the Earth for the other part of the year. A flat surface captures more of the Suns rays than a tilted one. Therefore in the summer the Northern Hemisphere receives the rays of the Sun most directly while in the winter it receives them less directly and so it is colder. This is also enhanced because the tilt of the Earth means that in summer we not only receive the Sun's rays more directly but for a longer period of time. In the winter not only do we receive less direct sunlight, but we receive sunlight for less time because the tilt causes winter days to be shorter. The tilt is also responsible for the seasons being reversed in the Northern and Southern Hemispheres.