Sunspots are regions of strong magnetic field on the sun's photosphere. They appear as dark areas on the sun's surface (the umbra) surrounded by a less dark penumbra. They may persist for days or even months.
The frequency with which they occur varies in an eleven year cycle. At the beginning of the cycle they appear at latitudes of +/- 30o and subsequently progress towards the solar equator.
However, this cycle is not always followed and between 1645 and 1720 it stopped altogether (a period known as the Maunder minimum). During this time there was a mini ice age in northern Europe.
This type of irregularity in the sunspot cycle may explained by chaos theory.