A burr grinder (sometimes referred to as a Burr Mill) is a device that is used to grind coffee. A burr grinder uses rotating flat or conical metal/ceramic disks with sharp ridges (or burrs) to evenly grind the coffee beans. 

The advantage of this type of grinder over a “whirly blade” grinder is that the grains of coffee are all ground to a very consistent fineness. This is very important when trying to pull a good shot of espresso. As well, good burr coffee grinders will grind at a low enough speed to keep the beans from overheating and disrupting its taste profile.