Also what are calipers? In the sense of what your referring I use them for?
Calipers are a device used to measure the diameter of either a shaft (using 'outside calipers') or a hold ('inside calipers'). Actually, most calipers today can do both inside and outside dimensions.
Calipers come in three varieties - analog calipers have a simple calibrated scale engraved on the body of the caliper, and a pointer that tells you the dimension of the shaft (when the calipers close down on a shaft), or hole (when the caliper expands to the diameter of the hole). And most importantly, they have a locking screw so that you can measure the inside diameter of a hole, lock the caliper, and use the outside diameter measurement as a gauge when you turn a shaft to match. That is, the potential inaccuracy of an analog measurement doesn't matter because rather than reading the measurement, you use the caliper to simply transfer the measurement from one piece to another. The accuracy of a vernier caliper depends on its length - a typical 6-8" caliper is probably good down to 1/16", but a longer (and more expensive) caliper can be used to get more precise measurements.
There are special versions of analog calipers called 'vernier' calipers - these have two scales, one to read the total dimension, and a second that helps you interpolate between markings on the main scale. It takes time and experience to learn how to use a vernier caliper.
Dial calipers have a dial and moving needle - and so are more accurate than analog calipers in when used to make measurements. But they are no better than analog calipers if you use them to transfer measurements. A 6-8" digital caliper typically can read to three decimal places in either metric or decimal imperial measurements.
Then there are digital calipers - calipers with a digital readout, often one than can be switched between metric, fractional imperial, and decimal imperial. The measurement is more precise, but the key feature is the ability to transfer measurements without actually reading the dimension. The major downside of digital calipers is that they eat batteries.
Better calipers are made of steel, but you can buy less expensive plastic calipers. A plastic analog caliper will cost about $5, while a steel digital calipers start around $15 but can be far more expensive depending on the quality.
While calipers are intended mainly for measuring diameters of shafts and holes, most can also measure the depth of a hole.
Quality varies considerably, but for wood turning you don't need the same quality that you might want in a precision metal shop. Harbor Freight has fairly inexpensive plastic and steel analog calipers, and their steel digital calipers aren't all that bad. Brands like Starret are at the other end of the scale and are very expensive.