Sunspots are a feature of the Sun that have been observed since ancient times. When viewed through a telescope, they have a dark central region known as the umbra, surrounded by a somewhat lighter region called the penumbra. Sunspots are dark because they are cooler than the surrounding photosphere. They are the site of strong magnetic fields. The reason sunspots are cool is not entirely understood, but one possibility is that the magnetic field in the spots inhibits convection underneath them.
Sunspots typically grow over a few days and last anywhere from a few days to a few months.
The number of sunspots on the Sun is not constant. While the cycle has been relatively uniform this century, there have been large variations in the past. From about 1645 to 1715, a period known as the Maunder minimum, apparently few sunspots were present on the Sun and colder weather resulted on Earth. Although the number of sunspots is the most easily observed feature, essentially all aspects of the Sun and solar activity are influenced by the solar cycle. The Sun is not a quiet place, but one that exhibits sudden releases of energy.