The spots are simply cooler and darker than the surrounding hot and bright material. Their temperature is around 3,800 K instead of just under 6,000 K for the average photospheric temperature. The Sun is a great big ball of plasma and it has a magnetic field like Earth. Plasma is electrically charged gas that can drag magnetic field lines with it. As the Sun rotates it drags the magnetic field with it causing it to get wound up and tangled. Often the stress the field lines are under are so intense that they burst through the surface, inhibiting convection, making the temperature in this region cooler, the sunspot. The sunspot (and general solar activity) peaks over an 11-year cycle.