There are two types of putters: those that have a milled face and those that use some type of insert in the clubface.
Milled putters display a complex milled pattern on the clubface where the design attempts to produce the best feel and sound.
They often cost more than clubs with inserts, but they are thought to be worth it because they offer more precision and control. Some players believe that milled putters provide a better connection with the ball, resulting in increased accuracy.
Many pros use milled putters, including Phil Mickelson, Bubba Watson, and Zach Johnson.