A sun spot is an area of cooler plasma on the sun's photosphere. This cooler area is roughly 3000-4800 kelvins. These sunspots are caused by magnetic field lines that pop up through the sun's photosphere. Normally the magnetic field lines on the sun are in the convection zone, but if they twist enough they can kind of poke up through the photosphere. This is much like twisting a rubber band until it pokes up in the middle. Sunspots usually occur in pairs. One in the beginning of the magnetic loop and the other is the end. Plasma from the sun gets trapped in these loops causing many different kinds of solar phenomenon.