Here is a web page with working micrometers and calipers.
As for the working parts..?
Vernier: jaws (put object between, or outside for inner msmnts)
Main scale, reads to nearest mm
Vernier scale reads to nearest .1mm

Micrometer
jaws: put object in between
coarse dial, to move the jaws quickly
fine dial/torque dial (small dial at end) only turns until jaws are a certain tightness, always applies same force to object to improve reproducibility.

Hope that helps

The name explains the micrometer clipers function, here is some history on it

"John Edison Sweet (October 21, 1832-May 8, 1916) was an American mechanical engineer who built the first micrometer caliper in 1873, for making tools, and who invented the “straight line” engine. He was a key founder of the American Society of Mechanical Engineers in 1880 and ASME’s third president (1884-85). Born in Pompey, New York, he died in Syracuse."

The advantage of the Venier Caliper is that it can measure not just inner distances but also outer distances with its two upper tines.