The Sun is the star around which our Earth revolves. It is the source of light, and thus the day is determined by the rotation of the Earth on its axis, putting any spot on the Earth's surface (between the Arctic Circle and Antarctic Circle alternately in the lighted area and the unlighted area. (North of the Arctic Circle and south of the Antarctic Circle, there are times of the year when this alternation does not occur; thise areas have periods when there is light all through the day and others when there is darkness all through the day.) (But see sidereal day, which is differently defined.)
The year is determined by the time of revolution of the Earth around the Sun, and different types of year depend on what is defined as the "same point" in the Earth's orbit around the Sun.