A sunspot is a dark region of the surface of the Sun. They are actually bright, but less bright than their surroundings, for being cooler than most of the Sun's 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 (solar variation). The Wolf number (aka sunspot number or Zürich number) is a measure of the amount of sunspots at a particular time. The frequency of sunspots and degree to which they cover the surface follows an 11 year cycle (solar cycle aka sunspot 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 details of the magnetic field: magnetic field lines pass vertically through them. The stronger magnetism impedes convection at that point, allowing the spot to cool.
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, and EMR is brightest and the Earth climate is warmest during the solar maximum.
Other stars can have analogous starspots (stellar spots) 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.