A watch movement, also known as a "caliber", is the internal mechanism of the timepiece which drives all of the timekeeping functions and is the essential component for keeping accurate time. Apart from digital watches, all movements fall under one of two categories: quartz or mechanical.
Quartz watches are battery powered, very accurate and reliable, and require minimal maintenance apart from occasional battery replacements. A small quartz crystal is electrified by the battery, creating vibrations that drive the motor to move the watch hands, essentially converting vibrations into time. This is the most accurate type of movement, keeping accuracy within half a second per day. An indication of a quartz movement is the 'tick' of the seconds hand, at one 'tick' per second, whereas a mechanical movement creates a 'sweeping' effect of the seconds hand.
Rather than using a battery for its power source, a mechanical watch contains an intricate series of mechanical components and harnesses the energy from a wound metal spring (mainspring). A precise, regulated release of the stored energy is then transferred through a set of gears to turn the watch hands and power the watch's functions. The elaborate engineering and high level of quality and craftsmanship are the characteristics which make these timepieces attractive and valuable to many watch enthusiasts, as they are not quite as accurate as a quartz movement. There are two sub-categories of mechanical watches, manual and automatic, which determine how the mainspring is wound.
Often referred to as "hand-wound", manual timepieces are manually wound by the hand to create energy in the watch's mainspring. How often the wearer needs to wind the watch depends on its power reserve capacity. Alternatively, automatic watches are self-wound via the wrist's motion during wear. As long as the watch is worn regularly, it will maintain power without requiring winding.