Electrical circuit overloads happen when more amperage is put across an electrical wire or circuit than it can handle. For instance, a #14 wire can safely carry 15 amps and should be protected by a 15-amp breaker. If it happens to get connected to a 20-amp breaker instead, the breaker will allow 20 amps of current to flow through a wire that can only handle 15 amps. The wire and breaker start to heat up and could cause start an electrical fire.
Ampacity can be calculated by dividing the amount of wattage by the voltage. Let's say you have a 1,000-watt hair dryer that runs on 120 volts. Divide that out and you'll get 10 amps of current draw. On a 20-amp circuit, that's half of the circuit used already.
Now, think about having a meal at your house. You plug in a couple of crock pots that each draw 1,000 watts of power and you can see how the circuit is at maximum capacity. The problem is, that likely there are other things plugged in also and undoubtedly the circuit will trip. In this case, the only way to solve the problem is to plug one of the devices into another outlet on a different circuit.
Circuit overloads can also be caused by loose or corroded wires and connections. This could be a breaker connection, a splice in a box, especially if the connection has been exposed to moisture, it could be a wire is not making good contact under a wire nut splice, only because the wire were not lined up evenly when installed, or it may be a loose connection on a part of a light fixture. Of course, the list goes on and on. The point is, if the breaker trips or the fuse blows, there is a problem. It may be a short or just a circuit overload.
Correcting the problem may be as easy as unplugging a device that you plugged into a circuit already loaded up. Just use common sense when there is a problem. Think to yourself, what did I just recently add to the circuit that tripped. Was it something that could have caused the problem? Maybe it's just something simple like a hot day and you plug in an air conditioner and a couple of fans. All of a sudden the breaker trips. Unplug the fans and wait a few minutes before resetting the breaker. Then, rest the breaker and try turning on just the air conditioner. Chances are that if you look at the nameplate rating on the unit, you'll see that the air conditioner used up most of the circuits and the fans put it over the edge.