What exactly is a short circuit?
In simple and practical terms, a short circuit is an unwanted or unintentional path that current can take which bypasses the routes you actually want it to take. 

This is normally a low resistance path between two points of differing potential.

For instance:







simulate this circuit &ndash; Schematic created using CircuitLab

In the left simple LED circuit, just over 6 mA is flowing round the circuit. Create a short circuit, represented by a very low resistance (no wire is a perfect 0 Ω conductor) and 5000 A wants to try to flow through it. That's bad news for the battery. The battery could well explode. What is certain, though, the internal resistance of the battery will limit the current that can exist and a large voltage drop will be seen at the terminals of the battery causing the whole circuit to stop functioning.