Are you overloaded? Have you become a part of highly stressed modern working environment? You may need a break - a vacation, a change in environment, a change of pace...
Similarly, machines can be overloaded too. Those that were designed for a particular system are often pushed to their limits when new requirements emerge.
Just like final straw that breaks camel's back, electrical machines too, can reach their breaking point. If there is no vacation for them, they will burn.
Fuses and circuit breakers are designed to stop current in a circuit before any damage can occur - create a vacation or a break for them.
A circuit breaker is one of ways to prevent motors (or other electrical machines) from burning due to overloading. All electrical motors produce heat when running. At time of a fault, this heat builds up at a much higher rate than it can be dissipated. This can result in damaged equipment, fire or even explosion.
The electrical protective device like circuit breaker is designed to de-energize circuit before this heat level becomes dangerous.
There are basically 2 means of activating a break - thermally (heat) or magnetically.
Thermal circuit breakers make use of bimetallic strips that will bend due to different expansion rates of each of metals in strip. The bending of strip will move a tripping mechanism latch that will then open up contacts of circuit breaker.
Because bimetallic strips take time to heat up and bend, this type of tripping device is used for tripping prolonged overloaded situations. The setting for breaking circuit at a particular overload value can be adjusted.