Its basically due to the position of the sun and the earth in relevance to the sun. The Earth is continuously revolving around the sun and at the same time it is revolving around its own axis. The part of earth that is facing the sun at a particular time is experiencing what we call Daytime. Whereas the part of earth opposite to that part is experiencing Nighttime. Whenever a part of earth is about to face the earth it experiences Sunrise. And when it is about to leave the part of earth and move ahead, the time is called Dusk.