The surface of the Sun - Solar Spots
Solar spots are areas on the Sun where the magnetic field is much higher than elsewhere. This increases the `magnetic pressure', which in turn decreases the normal atmospheric pressure. And because of that, the temperature in the solar spots is lower than in the surrounding area.
This makes them look like a dark spot. Click on the image for a larger view.
The number of spots on the Sun isn't constant, but varies with a period of 11 years, called the solar cycle. At the moment the number is in a minimum. This solar cycle has influence on earth.