A micrometer is composed of:
The C-shaped body that holds the anvil and barrel in constant relation to each other. It is thick because it needs to minimize flexion, expansion, and contraction, which would distort the measurement.
The frame is heavy and consequently has a high thermal mass, to prevent substantial heating up by the holding hand/fingers.
Explanation: if you hold the frame long enough so that it heats up by 10°C, then the increase in length of any 10 cm linear piece of steel is of magnitude 1/100 mm. For micrometers this is their typical accuracy range.
Micrometers typically have a temperature specified, at which the measurement is correct.
The shiny part that the spindle moves toward, and that the sample rests against.
Sleeve / barrel / stock
The stationary round part with the linear scale on it. Sometimes vernier markings.
Lock nut / lock-ring / thimble lock
The knurled part (or lever) that one can tighten to hold the spindle stationary, such as when momentarily holding a measurement.
(not seen) The heart of the micrometer, as explained under "Operating principles". It is inside the barrel. (No wonder that the usual name for the device in German is Messschraube, literally "measuring screw".)
The shiny cylindrical part that the thimble causes to move toward the anvil.
The part that one's thumb turns. Graduated markings.
(not shown in illustration) Device on end of handle that limits applied pressure by slipping at a calibrated torque.
source : http://en.wikipedia.org/wiki/Micrometer#Parts