Inside every watch, no matter the power source, there is a little engine that is delivering information to the display. This is referred to as either the "movement" or the "caliber," with both terms referring to the same thing.
Generally speaking, there are two types of watch movements: mechanical movements and battery-powered movements. You may also hear of "capacitor-powered" movements, but a capacitor is really just a rechargeable battery, which delivers power to the mechanism in exactly the same fashion as a traditional battery.
The distinction between these movements is simple: the wheels in these watches are driven by different power sources. In mechanical watches, the tension of a powerful coiled spring trying to unfurl is what drives the wheels forwards, whereas a battery-powered watch is driven by an electronic impulse that is released every second to advance the wheels accordingly.
In the earliest days of watchmaking, only mechanical watches existed. In the late 1700s, the craft was seen as a magnificent art form, with Kings and Emperors holding watchmakers in high esteem. As with any technology, however, mechanical watchmaking eventually became commonplace and affordable for the common man.
By the twentieth century, wristwatches became popular in a world that was becoming increasingly busy. This had a huge effect on watchmaking and the movements powered everything from the cheapest quartz watch to the most opulent super complication.
Two World Wars left their mark on the watchmaking industry, as accuracy took precedence when it came to executing military strategy and working as a coordinated unit across distances. Watches also needed to be able to withstand heavy shocks and maintain performance when exposed to extreme temperatures or magnetic fields.
Magnetic fields, in particular, became a problem for mechanical watches as the century progressed. New technologies brought with them larger, more powerful magnetic fields. Mechanical watches are particularly susceptible to magnetism, as timekeeping is regulated by a very fine spring called the hairspring or balance spring. Anti-magnetic cages (called Faraday cages) were added to certain watches that were likely to be worn by people coming into frequent contact with heavy machinery or scientific equipment that may be highly magnetic. Some watches went even further by engineering the entire movement out of anti-magnetic materials. The most famous of these is the Rolex Milgauss, which was specifically developed for scientists. Ultimately, these advancements would succumb to the popularity of the quartz watch two decades later.
In the 1970s, the next generation of timepieces arrived. Emerging in Asia, battery-powered quartz movements were incredibly accurate and easy to produce in comparison to the labor-intensive mechanical variety. Although initially expensive, quartz technology quickly became very, very affordable. Its popularity nearly destroyed the mechanical industry entirely, and it was only through the astute rebranding of traditional watchmaking as an art, rather than a necessity, that the age-old craft was able to survive at all.
Now, both mechanical and battery-powered watches exist side-by-side. The battery-powered variety has capacitor-driven movements that are charged by either an oscillating mass or sunlight. Time-keeping duties in these watches are handled by either a quartz crystal, a magnet-propelled flywheel, or a reverberating tuning fork. In mechanical watchmaking, it is possible to find manually-wound movements that have to be wound by hand every day so that they keep on running, or automatic watches that are constantly recharged by the movement of the wearer's wrist.
Quartz watches are normally cheap, fun, functional, and bought (mostly) by people who don’t want to worry about servicing or maintaining a mechanical timepiece every few years. Meanwhile, fans of mechanical watchmaking tend to be interested in a great many details of the craft, from the traditional to the avant-garde.
These nuances make studying watchmaking and all the different movements an endlessly exciting and interesting pursuit that is, thanks to the wide range of price points, more accessible than ever.