Imagine the inside of a wire...let's say that the wire is only 3 atoms across, like this:


These atoms all have electrons moving around themselves.  Some of these electrons are held very tightly (the ones close to the nucleus) and some are held loosely (the ones farther out).  We choose what we make wire out of based on how many of these special "loose" electrons a material has.  Let's say that in our wire, each atom has one "loose"  electron.  In my drawing, this electron is represented by the dark dot.

Notice that a few of these atoms are missing a dark dot.  This means that particular atom doesn't have any loose electrons.  I shade these in red so you can see them easier...

These red shaded atoms are called holes.  You can think of them as a place where there isn't an electron, but one could go there if there was one around.
What if we pump some electrons into the LEFT end of the wire?  The loose electrons in the neighborhood are repelled, and they move to the RIGHT.
Well, this means a current of electrons is flowing in the wire.  What happens when an electron moves into one of the "holes"?
It must mean that the atom where the electron came from must now be a hole.  So...if electrons move one way, the holes move THE OTHER WAY.  Please note that the actual atoms do not move (only the loose electrons are free to move).
 

if we pump electrons in herethen they come out down here!

>>>>>>> negative electrons >>>>>>>
<<<<<<< positive holes <<<<<<<