Parts

The visible components of a watch such as the bezel, the lume and the dial.

Bezel: The bezel is the ring around the crystal at the top of the case, framing the dial. It may be fixed, or it may rotate to measure dive time (a unidirectional diver's bezel), a second time zone (GMT) or speed (tachymeter).
Case: The case is the body of the watch that holds the movement, dial and crystal together. It largely defines the watch's diameter, thickness, water resistance and overall character. Cases come in shapes such as round, tonneau, cushion and square, and in materials like steel, titanium or gold.
Caseback: The caseback is the back cover of a watch case. It seals the movement inside and forms the surface that rests against your wrist. It can be solid or a display (exhibition) caseback, and it either screws down or snaps on. Its engravings usually list model, water resistance, and serial details.
Crown: The crown is the grooved knob on the side of the case. You use it to set the time and date and to wind a mechanical watch. Pushed in it runs and winds a mechanical calibre, pulled out it sets the time and date. A screw-down crown threads shut against a gasket for water resistance.
Crystal: A watch crystal is the transparent cover that protects the dial. Three materials are common: sapphire crystal, the hardest and most scratch resistant though costly; mineral glass, cheaper and more prone to scratches but better at resisting shattering; and acrylic, soft and scratch prone yet able to be polished out. An anti-reflective coating cuts glare.
Lug-to-Lug: Lug-to-lug is the vertical distance from the tip of the upper lug to the tip of the lower lug. It is this measurement, not the case diameter, that decides whether a watch will truly fit your wrist.
Lugs: Lugs are the four arms that extend from the case and hold the strap or bracelet. The gap between them sets the lug width, and how far they reach sets the lug-to-lug length. Together they shape how a watch sits on your wrist more than the case diameter alone.
Lume: Lume is the glowing material applied to the hands and markers; it charges in light and glows in the dark, letting you read the watch at night. The most common types are Super-LumiNova and Seiko's LumiBrite.
Sapphire Crystal: Sapphire crystal is the transparent cover over the dial made from synthetic sapphire. It rates a 9 on the Mohs hardness scale, second only to diamond, so it resists scratches far better than mineral glass, although it costs more to produce.
Screw-down Crown: A screw-down crown threads down onto a gasket to seal the case for water resistance. You unscrew it to set the time, then screw it shut again before any water. It is standard on dive watches, and forgetting to screw it down lets water into the case.