The sunspots are regions of the photosphere that are actually extremely high magnetic fields, due to the differential rotation rate of the sun and the bunching up of the magnetic field lines of the sun. The overall magnetic field lines run N-S, but as the equator of the sun rotates faster than the poles, the field lines get stretched out closer to the equator, eventually resulting in some pinching off. Recall that the strength of a magnetic field is proportional to the number of field lines/unit area, so the pinching off of the magnetic field lines of the sun results in locally very intense magnetic fields, that are a couple of million times as strong as the overall magnetic field of the sun, and also are approximately E-W instead of N-S. That is why sunspots are found in pairs: one is a north magnetic pole, and the other is a south.

So, at times of high sunspot activity, we get increased magnetic storms on Earth, and more interruptions to communications. Now, why are they cooler - that is because the extremely high local magnetic fields over sunspots prevent the charged plasma of the sun which is convecting underneath to rise up to the surface under the sunspots and release its heat.

So the convection cells get diverted under the sunspots, because the magnetic fields of the sunspots deflect the moving plasma, which has its own magnetic field. (Recall that charged particles that are moving, i.e., electric currents, create magnetic fields that are perpendicular to the direction of their motion and form closed loops.) So that is why locally the sunspots are cooler, and look darker. Also, they are cool enough that the hydrogen in the solar atmosphere above them can recapture its electron and become excited neutral hydrogen, and they look reddish because of the H-alpha emission.

However, high sunspot activity is also associated with lots of solar flares and, as I mentioned above, lots of charged particles streaming into space. The presence of lots of sunspots sends higher fluxes of "solar wind" -charged particles from the sun, that impinge on the earth and wreak havoc with our electromagnetic communications, and cause beautiful auroras.

But - to my knowledge - there is no verified correlation between climate change and sunspot activity.

During magnetic storms, more faculae (bright spots emitting increased heat) as well as more sunspots appear on the Sun. The effects of the faculae typically override the effects of the sunspots so that periods of magnetic storms correspond with periods of higher solar irradiance and higher insulation on Earth.

Sun spots are cooler areas of intense magnetism on the surface of the sun. Their darkness made them the first type of feature on the surface of the sun that was observable from Earth, so we tend to focus on them more. However, they are just one manifestation of the intense magnetic storms that rage on the surface of the sun in conjunction with the reversal of the sun's magnetic field every ~11 years. Bright spots called faculae have been recently observed and they abound near the magnetically active sun spots. These are so bright, that they actually cause the total sun luminosity to increase, despite the darkened sun spot regions.

National Geographic July 2004 has a great article (and of course fabulous images) about the sun if you want to learn more.

Sun spots are regions of intense magnetic fields on the sun's surface. Although they are small cooler regions, during periods of high solar activity, there are lots of sun spots and the solar average radiation from the other (surrounding) regions is much greater (radiated power goes as the 4th power of the temperature so that a relative small change in the temperature denotes a large change in the power output.)

Unlike many global warming measurements, the solar output is measured continuously by satellite and the correlation to sun-spot number is very well established.