Why do eclipses happen?

This diagram shows a side view. If you stand anywhere in the grey penumbra, you will see some part of the sun shining from behind the moon. The penumbra is the area of partial eclipse. Only from within the tiny area where the dark umbra touches the earth will you see the sun completely covered and experience a total eclipse.

This shows how the moon casts a shadow on the earth during an eclipse

The diagram below shows the path of the umbra for the eclipse on August 11th, 1999. Only the central lines mark out the path of the umbra. The much wider area shows the path of the larger penumbra, where a partial eclipse can be seen.

The middle line shows the path of totality, where people see a total eclipse. The other lines show the track of partial eclipses. The dotted lines show the time of day.