A sunspot is a dark region of the surface of the Sun. They are bright, but darker for being cooler than the normal surface. Some are easily seen with a simple projection of the Sun, e.g., through a pinhole onto a surface. They are temporary, lasting days to months.
The frequency of sunspots and degree to which they cover the surface follows an 11 year cycle (solar cycle) which coincides with the time between reversals of the Sun's magnetic field (giving the Sun a 22-year cycle between reversals identical in polarity). The spots themselves are related to finer detail in the field, having magnetic field lines passing vertically through them.
The period within the 11 year cycle when sunspots are most frequent is termed the solar maximum and least frequent, the solar minimum, which also coincide with other solar phenomena including slight changes to the Sun's luminosity (i.e., affecting the solar constant and insolation). When sunspots face the Earth, they reduce the electromagnetic radiation toward Earth, but the Sun is otherwise-brighter during the solar maximum, more than making up for the reduction over the longer run, and solar maximum is when EMR is brightest and Earth climate is warmest.
Some other stars have analogous starspots which can be detected by changes in apparent magnitude, and in the most pronounced cases, are the mechanism behind some types of variable stars. They are useful for stellar rotation determination. Their possibility must also be taken into account when identifying transits attributed to binary stars or extra-solar planets.