A chain is a series combination of linked parts, called a link, typically made of metal, flexible and curved in compression, but linear in tension, rigid and having the general character similar to a load-bearing rope. A chain can consist of two or more links. Chains can be classified according to their design, which can be dictated by their use:

Designed for lifting, such as when used with a crane; to pull; or to fix it as in a bicycle lock, it has donut-shaped links that make the chain flexible in two dimensions (the fixed third dimension is the length of a chain)

Small chains that act as jewels are mostly a decorative counterpart of this species.
Those designed for power transmission in machines have connections designed to engage the teeth of the machine's gears and are flexible in only one dimension. Even though they are non-roller chains like block chains, they are known as roller chains.