Electric currents in the sun generate a magnetic field just as they do in the earth. Where magnetic field lines exit the sun's surface, they generate a magnetic disturbance in the visible surface of the sun. A sunspot appears dark because the temperature there is much lower than it is elsewhere on the sun's surface. Sunspots can grow in size from the time they appear to the time when they disappear, usually in weeks or months.

The sun is made of plasma (ionized matter). < Because it is ionized, all of the particles in the sun are charged, and anything charged is subject to electric and magnetic fields. The movement of charged particles, which in the sun's case represents the boiling convection releasing heat from the interior, also creates magnetic fields.

The effect of the sun's magnetic fields on the surface is to suppress and concentrate some of the plasma on the surface and to throw some of it off into space (solar wind, solar storms, etc.). The relatively cool spots are sunspots.

I did a Google search on this topic and found the following answer:

Look to Stan Odenwald's "Ask the Astronomer" for this one.
The Astronomy Cafe

Why are sunspots dark?
Sunspots are a magnetic phenomenon on the sun. You can think of them as small pores on the surface of the sun where lines of magnetic force enter and exit. Sunspots always come in pairs like the north and south poles on a bar magnet. The strength of the sunspot magnetic fields are usually 1000 times as strong as the average solar magnetic field. Because magnetic fields can produce pressure, inside sunspots, the gas does not need to exert quite as much pressure as elsewhere on the sun to insure that the total pressure across a sunspot is in equilibrium with the gas surrounding the sunspot. Since the cooler a gas is, the less pressure it exerts, this means that the gas inside a sunspot can be cooler than the gas in the rest of the solar surface and still, with the help of the sunspots magnetic field, remain in equilibrium.

The average gas temperature of the solar surface is about 6050 K, but inside a sunspot, the gas temperature is only 4200 K. The reason a sunspot appears dark is that the gas inside the spot where the magnetic field is strongest is only emitting about 1/4 as much light as from the rest of the solar surface. If you were to rip a sunspot out from the solar surface and put it in the night sky, it would appear as a bright, orange gas, not a dark void.