A chuck is a device used to securely hold an object in place. An example of this might be the screwed metal sleeve that surrounds a drill bit in an electric drill. Collet chucks are a little more specific in that they hold wood or metal cylinders in place for shaping via lathes.
Standard Collet Chucks
The collet chuck consists of a round flat backplate with either four or six arms rising up around the plate's circumference. Two chucks are typically used at once, one at either end of the material which is to be shaped. The arms tighten down against the material, which is slipped between them by outward facing screws and push the arms inward slightly. It should be noted that these arms have a very limited range as they are integral to the backplate and could be broken if pushed too far inward. For that reason, the circumference of the material must be very close to that of the collet chuck. The backplate of each collet chuck extends in a threaded screw, which is screwed securely into the spindles of the lathe before the lathe is turned on.
Spring Collet Chucks
Spring collet chucks are a time-saving variation of the standard collet chuck. Instead of having a backplate and arms, the chuck is a hollow cylinder, much like a sized attachment for a socket wrench. The sleeve is split lengthwise into four quarters which have a high tensile strength. A length of wood or metal can be slid into the sleeve and held in place by this tensile strength, doing away with the need to screw the arms of a traditional collet chuck down tight. Again, two spring collet chucks are used in tandem, each screwing into the end of a lathe's spindles. And again, because the sleeve of the spring collet chuck is a very specific circumference, the size of the material being worked should be close to matching, necessitating many different sizes of spring collet chucks for different jobs.