A sunspot is a temporary cool region in ‘s photosphere (the visible surface of the Sun) associated with an active region with a magnetic field intensity of a few 0.1 T.
Sunspots appear darker than the surrounding photosphere (at about 5800 K) because they are cooler (about 3800 K). The darkest central part is called the umbra which is normally surrounded by the lighter penumbra with a radial filamentary structure.
The most important characteristic of a sunspot is its magnetic field. Typical field strengths are near 0.1 T (up to 0.4 T). These fields inhibit the convective transport of energy (hot gas rising from the convective zone of the Sun) reducing locally the temperature.
Basically the sunspot and its main parts are a sheaf of magnetic flux tubes filling the umbra and the penumbra and fanning out above them:
The lifetime of a sunspot ranges from a few days (small spots) to months (large spots).
Sunspots appear as a cyclic phenomenon; successive sunspot maxima (or minima) occur every 11 years on the average. A new cycle begins when the number is a minimum.
Sunspots are normally localized at high latitudes (##+-35##) at the start of a cycle getting to ##+-15## at maximum and to ##+-8## at minimum of the cycle (with very few sunspots found at latitudes greater than ##+-40##).
(Pictures and data reference: M. Zeilik S. A. Gregory E. v. P. Smith Introductory Astronomy and Astrophysics Saunders College Publishing1992).