A sunspot is a visibly darker area, appearing in the photosphere of the sun. They are a temporary phenomena, each individual sunspot having a longevity that can vary from a few hours to a couple of months. The spots appear darker than the rest of the photosphere because they are at a lower temperature.
The average temperature of the sun's photosphere is 5800K, (degrees Kelvin) and it is in a constant state of flux due to large convection currents bringing up material from the interior. Sunspots are associated with areas of intense magnetic activity. It is this magnetic activity which temporarily stops convection and thus results in areas of reduced surface temperatures. Sunspots have temperatures in the range 3000 - 4500K, which is still intensely hot. If we could view an isolated sunspot it would appear brighter than an arc-lamp. It is only relative to the much hotter photosphere that they appear so dark. As the radiant heat of a body is proportional to its absolute temperature raised to the power of four, we see the reason for the disparity in colour between the two regions.
The largest sunspots are in the region of 80,000 kilometers in diameter and are clearly visible from Earth. They usually appear in pairs and the pairs in groups. Pairing is due to the magnetic nature of their formation, each member of a pair having the opposite magnetic polarity. Sunspot size is not constant, and they will expand and contract during a cycle of magnetic activity. They also move across the surface of the sun, with speeds in the region of a few hundred metres per second. The increased magnetic activity associated with sunspots also gives rise to phenomena such as coronal loops and solar flares.
The intensity of sunspot activity varies over an approximate 11 year cycle. The number of sunspots also varying from cycle to cycle. The number of sunspots also correlates with the intensity of solar radiation. Even though the the sunspots themselves are cooler, their margins are intensely hotter than the rest of the photosphere and so overall more heat is radiated by the sun. Thus a more active sun means a warmer earth. The so called "Little Ice Age" of the mid-to-late seventeenth century, when rivers remained frozen and there were very harsh winters, coincided with a period of little or no sunspot activity, known as the " Maunder Minimum" (1645-1717).
Today most observations of solar activity are done in orbit using solar telescopes. To directly observe sunspots filtration and then projection is used. The amateur observer must never look directly at the sun for fear of damage to the eyes, a point well worth making in any article on the subject. Since 1979, satellites have been used to measure the absolute radiation emitted by the sun and the correlation with sunspot activity is now beyond doubt. As research continues evidence has emerged for larger cycles of activity within the sun, of which the 11-year sunspot cycle is but a part. An article such as this is merely a basic overview of sunspots and the reader is encouraged to further investigation.