Z is for overclocking, H isn't. The price between a H77 and a Z77 is quite close to each other, so people tend to go for the Z one to have that extra feature for later use. People who don't have the desire to OC often go for Z77 over H77, too, because Z77 motherboards often got more high-end features from manufacturers over H77, like SLI support, high-end sound chipset, DEBUG LEDs, etc.