Cables

Some helpful clues:
Feedback is caused by the mic picking up its own sound from the speaker. This may be because the speaker is facing the mic, or because the sound is bouncing around the walls and stage back into the mic. Sound is made up of many frequencies. This is why feedback sounds like a steady 'tone'. When feedback starts you hear a 'ringing' sound at that frequency, usually enough warning to act fast and pull the level back. If you let it continue it grows to become a shrill tone that can reach maximum PA level if left unchecked. It can be quite effective in clearing the hall!