Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness. Observed starspots are in general much larger than those on the Sun, up to about 30 % of the surface may be covered, corresponding to a size 100 times greater than the ones on the Sun.
To detect and measure the extent of starspots one uses several types of methods.
Observed starspots have a temperature which is in general 500-2000 Kelvin cooler then the stellar photosphere. This temperature difference could give rise to a brightness variation up to 0.6 mag between the spot and the surrounding surface. There also seems to be a relation between the spot temperature and the temperature for the stellar photosphere, indicating that starspots behave similarly for different types of stars (G-K dwarfs)