An eclipse happens when the Earth, the Moon and the Sun are in direct line. A lunar eclipse happens when the moon passes through the shadow of the Earth and the Moon becomes relatively dark in the sky. A solar eclipse happens when the Earth passes through the shadow of the Moon and the sunlight is partially or totally cut off. During a total solar eclipse the disk of the Moon covers the Sun completely and the part of the Earth in the path of totality is as dark as night. Total solar eclipses happen in restricted areas and totality never lasts more than a few minutes.
 Non-mainstream explanations
The International Flat Earth society believes that the sun and moon are only a few miles across and are only a couple hundred miles overhead. Furthermore, they believe that neither the sun nor the moon ever actually dips below the horizon -- the apparent rising and setting are due to tricks of perspective. Lunar eclipses, in their model, are caused by an unseen dark body occasionally passing in front of the moon.