Automated External Defibrillators (or AED's) are intelligent devices that analyse the heart rhythm and provide an electrical shock to the heart if someone is in cardiac arrest. Defibrillation, paired with good quality CPR, is the only way to treat someone who has had a cardiac arrest.
Contrary to popular belief, the only harm that can be caused by a defibrillator is if anyone is touching the casualty when a shock is delivered - it's worth remembering that the device will analyse the heart rhythm and won't deliver a shock if someone isn't in cardiac arrest. You also don't have to be trained to use one, but it does help to make you feel more confident to step in and save a life if you are trained.
Automated external defibrillators (AEDs) can either be semi-automatic or fully-automatic. The main difference is that a semi-automatic defibrillator has a button for the rescuer to push if the device determines a shock is required, whereas a fully-automatic defibrillator will automatically shock the patient if a shockable heart rhythm is detected, ie. user intervention is not required to actually administer the shock. Semi automatic devices are generally regarded as safer for untrained people to use, as there is less chance of you or any bystanders getting shocked if you're touching the patient when a shock is delivered.
Regardless of the make and model of defibrillator, they are all designed to be used by anyone. They give you full audio and visual instructions to help guide you through the rescue process. You will be surprised at just how easy they are to use!