Galileo sunspot sketch
Sunspot observations were hit and miss for the next 150 years or so but from 1749 systematic observations have continued to the present day (today satellites observe the Sun continuously). These observations have shown that the Sun has a roughly 11 year sunspot cycle and we're currently moving into a solar cycle 24 (the 24th cycle since 1749) and we'll start to see increasing numbers of sunspots over the next few years.
Cycle 24 officially began on January 4, 2008 with the appearance of a sunspot with reversed polarity from those of the previous solar cycle 23. The sunspot cycle is tied to periodic reversals of the Sun's magnetic field of 22 years. Sunspots form because the Sun's magnetic field gets "knotted up" in places (due to differential rotation in a ball of gas) which inhibits convection from the interior resulting in a cooler, darker-appearing area on the surface of the Sun. "Cooler" is relative, however, the Sun's surface temperature is over 9,900° F (5,780 K) and sunspots are a chilly 4,940–7,640 °F (3,000-4,500 K). They just appear dark because the surrounding area of the Sun is so bright. Here's an easy-to-read introduction.
Sunspots typically last days to weeks and are associated with coronal mass ejections (CMEs) and solar flares. Flares release powerful bursts of electromagnetic radiation moving at the speed of light while CMEs are eruptions of plasma (charged particles) from the Sun which "only" move around 500 km/s (on average). While electromagnetic radiation from the Sun travels at 300,000 km/s and takes 8 1/2 minutes to reach the Earth, the charged particles from CMEs take a few days to get here.
The Sun always emits a stream of charged particles towards the Earth, we call it the solar wind, but CMEs increase the amount by orders of magnitude. Normally, the solar wind is deflected away from us by the Earth's magnetic field, but when a CME occurs, the charged particles can leak into the Earth's magnetic field at the polar regions and ionize nitrogen and oxygen molecules in the upper atmosphere causing them to glow red and green (typically) - the Aurora Borealis (Northern Hemisphere) and Aurora Australis (Southern Hemisphere). While best seen at high latitudes (like Northern Europe or Alaska), auroras can be seem right here in the Hudson Valley if conditions are right (I saw a couple here during the last solar maxima back around 2000/2001).
This is all a gross simplification, by the way, the details are much more messy (and it's still a very active area of research among solar physicists).
Anyway, you may have noticed the solar image on the sidebar of my blog. This image is updated every day from the Solar and Heliospheric Observatory (SOHO) satellite (click on the image for more details). A new sunspot appeared a few days ago called sunspot 1092 (they're each assigned numbers). It moves as the Sun rotates so here's a screen capture from August 2 in case you're reading this in the future!
At 0855 UTC (4:55 am EDT), a large solar flare and CME occurred from sunspot 1092 and the charged particles are heading directly toward the Earth. It will arrive tomorrow sometime (August 3) and may result in some auroral activity. Go outside when it's dark, away from artificial lights, and take a look. You never know. Worst case scenario? Enough charged particles arrive and penetrate the magnetic field to blow out communication satellites and bring down the power grid (read about what happed in solar cycle 22 in 1989). [No, I don't think that will happen tomorrow - but it may eventually!]
Read more, and look at some cool movies of the Sun, at a great website - SpaceWeather.com.
Sunspot 1092 visible in rising Sun (lower right - image reversed from those above)