What's a burr grinder?

Domestic coffee grinders are one of two designs.  They are either a spinning blade (propeller shape), like an upside down lawnmower, or a tapered male and female burr design.  A blade grinder will simply smash the beans up, rather than cut them, and as a result the grinds will vary greatly in size.  While you can get away with a blade grinder for making filter coffee, it will simply not suffice for espresso.  Yes, burr grinders cost substantially more, but they require an electric motor that is a lot more powerful, and all the load bearing components need to be a lot stronger than just a propeller flying about.