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Calipers, which can be Vernier, dial or digital, are versatile tools for basic length measurement. They can be used on the shop floor, in the inspection room or even in the home by hobbyists.

Source: Caliper Tips And Tricks

A micrometer consists of two opposing surfaces, a stationary anvil and a moveable spindle. On most micrometers, these hardened steel or carbide-tipped contact surfaces are flat. However, micrometers can also be equipped with contact tips with unique forms for measuring special part characteristics.

Convenience is one of the reasons the micrometer is often the tool of choice for length/diameter measurements. The basic micrometer provides direct size information quickly, has high resolution and is easily adaptable to many different measurement applications.

Source: How to choose a micrometer that fits your needs

Gages are indirect reading instruments. The measurement units live not on the scale but off-site (for example, in a calibration room), and a master or other standard object acts as their substitute. The workpiece is compared against the master and only indirectly against the measurement units. The gage thus evaluates not the dimension itself, but the difference between the mastered dimension and the workpiece dimension. Examples of these comparative gages include an adjustable bore gage, mechanical snap gages or bench stands with dial indicators.

Gages tend to be faster and more accurate than measuring instruments. This is because they are dedicated to a particular size, and over their short range, they produce higher resolution and hence, greater accuracy.

Source: When Gaging Becomes Measuring That Becomes Gaging

A coordinate measuring machine (¸ßÅâÂÊÁùºÏ²Ê) is a measurement tool that consists of a motion system, a probing system, a computer and measurement software.

Source: Machining 101: What Are Coordinate Measuring Machines?