Very good question! And a difficult topic. The solar cycle is thought to arise ultimately from the Sun's dynamo process. During the solar minimum Sun's magnetic field is most simple, resambling a dipole field (like in the Earth's magnetosphere). The magnetic field of the Sun is generated in the convection zone (or rather in the boundary of it). Sun rotates faster at equator than at poles and this uneven rotation twists north-south orientated field lines around the Sun. The resulting magnetic tubes rise to the surface and one manifestation are sunspots. Large-scale solar flows (so-called meridional flows) closer to the surface transport magnetic field to the poles where it interacts with the existing field and changes it sign slowly. During this time when the north and south poles of the Sun are changing the Sun is most active (e.g. you see plenty of solar eruptions) and its magnetic field is most complex. Then situation returns back to solar minimum dipole configuration.

This cycle from minimum to minimum takes approximately 11-year. Why it is 11-years I do not know, but I have understood that at least speed of the large-scale flows that transport the magnetic flux to poles and back affect strongly to solar cycle length. For example, according to some models the changes in these flows would be responsible for the recent deep and long solar minimum. Also, if one looks sunspots statistics all cycles are not 11-years, it varies approximately between 10 and 13 years.

And to make things even more complicated it seems that Sun's south and north hemispheres do not evolve always in phase! You can read about this for example here: http://www.nasa.gov/missi...pole-asymmetry.html. So there is a lot we do not still understand about solar cycles.