The Earth's axis is tilted 23.5° to the east. As Earth moves around the sun, different parts of Earth point towards or away from the sun. This causes the different seasons.
When the north hemisphere is tilted towards the sun, it is summer there. When it is tilted towards the sun, it receives more direct sun rays, thus the heat.
When the north hemisphere is tilted away from the sun, it is winter in there because it receives less direct rays.
The northern and southern hemispheres always have opposite seasons. If the north hemisphere is tilted towards the sun, the southern one is tilted away.
The equator receives the highest number of direct rays throughout the year, resulting in a warmer temperature all year round. The poles, on the other hand, get no sunlight in the winter, but in the summer the sun never sets.