There are 5 basic types of dumbbells: Rubber-encased, Solid, Vinyl-coated, Neoprene-coated, and Adjustable.
Rubber-encased dumbbells are steel dumbbells featuring rubber-coated heads. The rubber coating protects gym floors and benches from dropped weights as well as protecting the weights from damage when stored on traditional metal racks. The handles on rubber-encased dumbbells are steel or chrome and typically knurled for increased comfort. This style of dumbbell has a wide range of available weights ranging from 2.5 lbs up to 125 lbs.
Solid dumbbells are very similar to rubber-encased dumbbells in general structure, but do not have the protective rubber coating around the heads. The heads and handles are cast separately and bonded together using molten iron. This style of dumbbell comes in cast iron, steel, and chrome variations. Handles can be knurled or straight and may be different lengths on different styles. Depending on the style, solid dumbbells can be found in weights from 1-150 lbs.
Vinyl- and Neoprene-coated dumbbells have a core of cast iron coated entirely with colored vinyl or neoprene. These dumbbells are very popular for group workout rooms in gyms; the weights usually have an individual color for each weight making them easy to recognize and organize. Another popular use for Vinyl-coated dumbbells is during aerobic exercise. When using weights during a walk, job, or run, you will want coated dumbbells. The coating over the handle improves the grip when your hands are sweaty and will protect the heads if accidentally dropped on hard running surfaces. The Vinyl coating is smooth and has a glossy appearance, while the Neoprene is textured and has a matte appearance.
Adjustable dumbbells include the handle, weight plates, and collars. The weights of the dumbbells can be changed between exercises by switching the amount/combination of weight plates. Always be sure to use the collars that come with the handles to ensure safe lifting.