An eclipse occurs when three celestial bodies become aligned. When the Sun, Moon and Earth align such that the moon is in the middle a solar eclipse occurs and when the alignment is such the Earth is between the Sun and Moon a lunar eclipse occurs.
A solar eclipse occurs when the moon moves across the disc of the sun and cast its shadow across the face of the Earth. The inner part of the shadow is called the umbra. None of the sunlight can penetrate the umbra and so to observers from Earth within the umbra the sun appears completely covered by the moon. This is known as a total eclipse. The outer part of the moon’s shadow is called the penumbra. To observers on the Earth on which the penumbra falls the moon appears to partially cover the sun’s disk and this is called a partial solar eclipse.
The distance between the Sun and the Earth is about 390 times the distance between the Earth and the Moon, and the Sun’s diameter is approximately 400 times the Moon’s diameter. Due to this ratio the Sun and the Moon when viewed from the earth appear to be the same size. It is for this reason that during a total solar eclipse the moon can totally block out the light from the Sun.
The animation below shows a solar eclipse.