Basic types
Large micrometer caliper
The image shows three common types of micrometers; the names are based on their application:
Outside micrometer (aka micrometer caliper), typically used to measure wires, spheres, shafts and blocks.
Inside micrometer, used to measure the diameter of holes.
Depth micrometer, measures depths of slots and steps.
Bore micrometer, typically a three-anvil head on a micrometer base used to accurately measure inside diameters.Tube micrometer, used to measure the thickness of tubes.
From http://en.wikipedia.org/wiki/Micrometer