Because the Moon is much smaller than the Earth, its shadow can only cover a small part of the Earth's surface, such that solar eclipses can only be seen from certain locations on the Earth. Although these rare events occur somewhere on the Earth about every 18 months, it's likely that it won't happen again in the same spot for another 300 to 400 years.
The Moon's shadow has an umbra (the dark cone-shaped shadow in the picture to the right) and a penumbra (the lighter shaded area). Someone in the umbra sees the whole of the Sun eclipsed, while someone in the penumbra sees only part of the Sun obscured.
These events are called a Total and a Partial solar eclipse respectively. A total solar eclipse only lasts for a few minutes, as the Moon's umbra moves eastward at over 1700 km/h. As a result, a total eclipse can never last more than 7 min 40 secs, and is often much shorter.