Sunspots are dark blemishes on the Sun's photosphere, or visible surface. They form where the solar magnetic field traps ionized gas and allows it to cool. Whereas the photosphere's average temperature is 5,700° Kelvin, in the dark center ("umbra") of a sunspot, where the field is strongest, it's about 2,000°K cooler. If you could view a sunspot by itself, it would appear blazingly bright. But in contrast with its even brighter surroundings, it appears dark. The umbra of a large spot is typically surrounded by a lighter penumbra, where the magnetic field isn't quite as strong.
The Sun turns on its axis roughly once a month, so this large spot will remain on the Earth-facing side of the solar disk until approximately July 10th. Remember, never look at the Sun without proper eye protection. There are a variety of ways to safely observe the Sun, many of which can be found in the solar observing section of our Web site. If it's cloudy at your observing site, you can check out the latest solar images from the Big Bear Solar Observatory.