A fuse or circuit breaker protects the wiring in an electrical circuit from allowing too much current to flow. A short circuit, for example, could be caused by two wires mistakenly crossed (a nail driven through the wall and touching two wires) that could cause a huge current flow and start a fire. Without fuses and circuit breakers, electrical circuits would simply catch on fire too many times for electricity to be considered a safe and practical energy to use. Since equipment will fail and wiring problems will happen, fuses or circuit breakers need to be included in circuits for safety.
Fuses work on the simple concept that when current flows through wire it generates heat, the more current flow, the more heat. The thin wire in a fuse will only allow a certain amount of current to run through it until it heats and disintegrates. The thin wire in the fuse is now gone and no current can flow the circuit. When current was flowing through the fuse and the rest of the circuit, it was a closed circuit, but when the fuse blows, it becomes an open circuit. No current flows in an open circuit. So fuses work well, but they only work one time. After the wire in a fuse burns out, that fuse must be removed and thrown away and a new fuse must be installed.
The circuit breaker accomplishes the same function as a fuse, but uses a simple switch to detect over-current situations. Therefore the circuit breaker can trip and be reset many times.