What are the fins or ridges along the sides of insulators for?

If you look up at a high voltage power line you will see that the cables are suspended from cool rods with fins along their lengths.  These weight supporting insulators are standoffs to prevent the electricity from arcing/short circuiting to the tower or pole.  You can determine the voltage on the transmission line by the length and number of these fins on the insulators, the higher the voltage the longer the insulator. 1 
 
The shape of the ridges increase the surface path length of the insulator between the wire and the structure holding it.  Electricity has a tendency to flow along the surface rather than through the air due to the accumulation of normal airborne contamination which causes a lower resistance than air.  They increase the spacing over the surface of an insulator and create a longer path for electricity to jump without increasing its physical length.  They also help with preventing the rain from falling in a continuous path along its surface and casing a short circuit.  The flashover voltage can be more than 50% lower when the insulator is wet. 2
 
Fun fin fact: 
The fins are actually called “sheds”, spacing through the air is called "clearance" and spacing over the surface of an insulator is called "creepage."3  A design with sheds greatly increases the creepage distance.