The visible surface of the Sun is called the photosphere. Sunspots are darker, cooler regions on the bright, hot photosphere. Whilst the photosphere has a temperature of around 6000°C, sunspots are between 3000°C and 4000°C.
Sunspots move across the surface of the Sun, expanding and contracting, reaching up to 50,000 km in diameter. They appear in pairs: one is a magnetic north pole, the other is a magnetic south pole. Magnetic field lines exit the surface of the Sun through one sunspot, and enter through the other. These strong magnetic fields prevent the process of convection.
Convection currents transfer heat, from the hot interior of the Sun, to the outer layers. In sunspot regions, convection cannot transport heat to the surface, and so sunspots are much cooler than their surroundings. Solar flares and huge solar storms (called coronal mass ejections) originate from sunspot regions.