Autoxfil is partially correct. Do a search of the site for more information. There are a few different types of helmets that are specific to both climbing and cycling. Some are hard shells with minimal styrofoam or padding and others are mostly styrofoam or padding with a thin shell.
Bike helmets, in general, are made for a single, large impact and then should either be retired or HAVE to be retired because they are in pieces. Climbing helmets, again in general, are made for several smaller impacts and dont need to be retired until the outer shell is broken.
I dont know about you, but my head is how I earn a living so I dont fuck around with being cheap with helmets.