A solar eclipse happens when the moon passes between the sun and the earth. Usually we can observe 2-5 solar eclipses every year.
If the moon covers only part of the sun when revolving in its obit around earth, it is a partial solar eclipse. A total solar eclipse is when the moon covers the sun completely, i.e. completely block the sun's light to earth. When a total solar eclipse happens, earth will receive no light or heat, and day will turn into night immediately. However, the complete darkness only lasts up to seven and half minutes because the sun is uncovered more and more as the moon moves on. The whole sequence of a total solar eclipse takes about two hours.