The Sun and the Moon appear to be of the same size when viewed from the Earth. So, when the Moon comes in front of the sun it appears to completely cover it. the shadow of the moon cast by the sun falls on the Earth, resulting in a total solar eclipse.
The sun, the Moon, and the earth must be in a straight line for an eclipse to occur. During a total eclipse, the Sun is seen as a black disc surrounded by a faint halo known as the corona. At any phase, before or after totality, the Sun is partly covered by the Moon, and appears as crescent. This is known as partial solar eclipse.
Sometimes, the Mon and the Sun do not appear to be of the same size. When the Sun is nearest to the Earth, and the Moon farthest from the Earth, the Sun will seem slightly bigger than the Moon. If an eclipse occurs at such a time, the Sun appears as a bright ring or 'annulus'. This is known as the annular solar eclipse.