Fuses and circuit-breakers are designed to break the circuit if current flow is excessive. The most common kinds are fuses, fusible links, and circuit breakers. They are all rated in amperes. Their ratings are usually marked on them.
Fuses are typically used in lighting and accessory circuits where current flow is usually moderate. Typically, a fuse contains a metal strip which is designed to overheat and melt when subjected to a specified excessive level of current flow, breaking the circuit and stopping the excessive current flow from potentially damaging more valuable components.
A fusible link is typically placed near the battery, and, except for the starter motor, it carries the current needed to power an individual circuit, or a range of circuits.
Circuit breakers are not destroyed by excess current. A bimetallic strip heats up and bends, opening a set of contacts and breaking the circuit. In most types, as the strip cools, it resumes its original shape. The contacts close, completing the circuit once more.