Hi. Can someone explain to me the purpose of the four diodes in an H-bridge circuit?

 

This should help:
http://en.wikipedia.org/wiki/Flyback_diode

 

Or try this ... which makes specific mention of the diodes.

http://www.modularcircuits.com/h-bridge_secrets1.htm

 

Hi. Can someone explain to me the purpose of the four diodes in an H-bridge circuit?

To freewheel the current during deadtime of an inductive load.