Detecting short circuits

What is a short circuit?
What is short to ground?

A short circuit is an electrical circuit that has unintended path to ground with no/low resistance. This usually happens when a component dies (e.g. Capacitors, MOSFETs, Resistors --- Resistors do not fail shorted, they fail open), creating a path to ground, whether that may be because of corrosion, blown capacitors, and so on.

Detecting shorts
Many times, finding short circuit could be obvious. First, Visually inspect the board! Other times, you might need to Inject voltage into the motherboard, because it would be harder to find shorts that have relatively high resistance to ground (higher resistance to ground = fewer voltage drop = harder to find short due to less heat generated).