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.
Both the orbit of the Earth round the Sun, and that of the Moon about the Earth, are not perfectly circular. At apogee the Moon has an angular diameter of 29.3 minutes — at perigee, 34.1'. At Earth's aphelion, the Sun has an angular diameter of 31.6' — at perihelion, 32.7'.
Here's a sort of ASCII-graphics visual aid:
29 30 31 32 33 34 minutes of arc
It follows that for almost half its orbit, toward the apogee end, the Moon is apparently smaller than the Sun. Under those conditions, a solar eclipse can never be total, and instead is, at most, annular—meaning that a ring of light is seen.
 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.