The document discusses different types of calipers - vernier calipers, dial calipers, and digital calipers. A caliper is a tool used to measure the distance between two opposite sides of an object. The vernier caliper, invented in 1631, provides more precise measurements than earlier calipers through the addition of a secondary vernier scale. Modern calipers like digital calipers offer additional convenient features such as automatic unit switching and data output. Calipers have movable jaws to measure outside dimensions and fixed upper jaws to measure inside diameters.