We get day and night because the Earth spins (or rotates) on an imaginary line called its axis and different parts of the planet are facing toward the Sun or away from it. It takes 24 hours for the world to turn all the way around, and we call this a day.
The earth's spin axis is tilted with respect to its orbital plane. This is what causes the seasons. When the earth's axis points towards the sun, it is summer for that hemisphere. When the earth's axis points away, winter can be expected. Also, Earth's tilted axis causes the seasons. Throughout the year, different parts of Earth receive the Sun's most direct rays.