Simply put, a cardiac arrest is when your heart stops pumping. This is often associated with a sudden collapse, as the blood supply to the brain ceases and the body shuts down. 
People often mistake these sudden drops to be a "heart attack".
A heart attack is simply a blockage in one of the arteries supplying the heart's musculature. A blockage like this can lead to a cardiac arrest, but it is not a cardiac arrest in itself. 

Anything that disrupts the heart's normal function to an extreme degree can cause a cardiac arrest. To learn more about the heart's normal function, please read our general overview.

An arrest may be caused by damage to the muscle, leading to an ineffective pumping action. This includes a heart attack, severe inflammation to the muscle, etc. 

An arrest may be caused by a disruption to the heart's electrical system. This includes electrolyte imbalance (particularly potassium and magnesium), a lack of blood supply to the septum (middle of the heart where the wires run through), severe whole-body infections, etc. 

When somebody arrests in hospital, we commence CPR immediately (chest compressions and breaths). We then study the heart's rhythm (electrical pattern) with a continuous ECG called telemetry - this helps us identify the type of arrest we are dealing with. 

Why is that important? Depending on the rhythm found, we may deliver a shock.
"Don't you shock everyone?" You ask. Unfortunately, it is not always appropriate...whatever 'Grey's Anatomy' says.

Shockable rhythms include ventricular tachycardia and ventricular fibrillation.
Non-shockable rhythms include pulseless electrical activity and asystole. 

Evidence favours delivering an electric pulse to the chest via pads in certain rhythms - essentially trying to jolt the heart out of the nasty circuit it's in. If we are successful in our attempt, we nickname this ROSC - Return of Spontaneous Circulation. 

Whether a rhythm is shockable or not, we give other medications such as adrenaline during a cardiac arrest, which acts as a chemical boost in a bid to kickstart the heart. 

During this whole process, chest compressions and adequate breaths (usually via a bag and mask) are delivered to the patient.
Meanwhile, we run other tests to try and uncover the cause of the cardiac arrest so that we may begin to correct it. 

As per the internationally recognised ALS guidelines, shocks are only given (if appropriate) once every two minutes. Unfortunately, the movies and TV shows lie to us when you see multiple, repeated shocks. However, I can assure you, the real deal can be every bit as dramatic; and we do say "Clear!"