Day and Night
Day and night are two times of day. The earth rotates once around itself in 24 hours. The side that is facing the sun is illuminated by it. There it is bright day. The other side is in darkness, there is night. The border between day and night constantly moves around the earth.
Sometimes "day" only means the time when it is bright. In this sense, day begins with sunrise and ends with sunset. The time of sunrise is also called "morning", the time of sunset "evening". The time in between can also be divided into morning, noon and afternoon. After sunset it is night until the sun rises again the next morning.
When you count the days, you usually mean day and night together. That makes 24 hours. This is how it is said that a week has seven days.