A watch is a timepiece that is to be worn on a person, as opposed to a clock which is not. The term now usually refers to a wristwatch, which is worn on the wrist with a strap, while a pocketwatch, the common type before World War I, is carried in a pocket and often has an attached chain to lift it out. Watches evolved in the 1600s from spring powered clocks, which appeared in the 1400s. In addition to the time, modern watches often display the day, date, month and year, and electronic watches may have many other functions.

The most common type of watch is the wristwatch, worn on the wrist and fastened with a watchband made of leather, nylon or other plastics (then called strap), metal links (called bracelet) or even ceramic. Before the inexpensive miniaturization that became possible in the 20th century, most watches were pocket watches, which had covers and were carried separately, often in a pocket and attached to a watch chain or watch fob.

In the 21st century, technological advances in metallurgy, composite materials development and physical vapor deposition greatly influence watch design and manufacturing. Solid stainless steel, titanium, tungsten carbide, carbon fiber, high-tech ceramic and ion plating processes dominate a considerable market share of today's modern watch-making industry. Sapphire crystals are often incorporated to complement and enhance the durability of a quality watch.

Most inexpensive and medium-priced watches used mainly for timekeeping are electronic watches with quartz movements. The most accurate watches have radio-controlled movements that are miniaturized, portable versions of radio clocks. Expensive, collectible watches valued more for their workmanship and aesthetic appeal than for simple timekeeping, often have purely mechanical movements and are powered by springs, even though mechanical movements are less accurate than more affordable quartz movements.

Purely mechanical watches are still popular, although they are most commonly seen among medium priced watches such as Fortis, Omega, Rolex and TAG Heuer and expensive watches like Patek Phillipe, Vacheron Constantin, Ulysse Nardin and Audemars Piguet. Their superb craftsmanship accounts for much of the attraction of purely mechanical watches. Compared to electronic movements, mechanical watches are inaccurate, often with errors of seconds per day. They are frequently sensitive to position and temperature, they are costly to produce, they require regular maintenance and adjustment, and they are more prone to failure.

Generally speaking, inexpensive and moderately priced timepieces with electronic movements now provide most users with timekeeping more accurate than the most expensive Rolex or Patek Phillipe. The most expensive, diamond encrusted Rolex contains a similar movement as its less expensive C.O.S.C rated brethren and all modern models can keep time to within 1 second a day. However, in recent times there has been less emphasis on one's watch for time precision as many people now carry multiple devices that will tell them the time accurately such as mobile phones, PDAs and laptops, these finely crafted mechanical watches have remained popular as precision time pieces and in many cases more so because of their aesthetic value as jewellery.

Wristwatches are often treated as jewelry or as collectible works of art rather than as timepieces. This has created several different markets for wristwatches, ranging from very inexpensive but accurate watches intended for no other purpose than telling the correct time, to extremely expensive watches that serve mainly as personal adornment or as examples of high achievement in miniaturization and precision mechanical engineering. Still another market is that of “geek watches”—watches that not only tell the time, but incorporate computers, satellite navigation, complications of various orders, and many other features that may be quite removed from the basic concept of timekeeping.

Most companies that produce watches specialize in one of these markets. Companies such as Breitling, Patek Phillipe, Jaeger-LeCoultre and Rolex specialize in watches as jewelry or fine mechanical devices. where as companies as Omega, and Tag Heuer all make mass-produced watches, mostly done completely with machines. Companies such as Casio specialize in watches as timepieces or multifunctional computers. Since watches are considered by many to be both functional and attractive, there are many types and manufacturers to choose from.

Important collectible American made watches from the early 20th Century were the best available at any price. Leading watchmakers included Elgin, Gruen, Hamilton, and Illinois. Hamilton is generally considered as having the finest early American movements, while the art deco styling of The Illinois Watch Company was unsurpassed worldwide. Early Gruen Curvex models remain very desired for how they entwined form and function, and Elgin made more watches than anyone else.