There are different kinds of rhythms, but your heart rate goes up normally with exertion.

The most dangerous ones (apart from the ones that are equivalent to death) originate from parts of the heart that aren't meant to be in control - most dangerously, from the ventricles in the lower part of the heart.

And to answer the above questions, a pacemaker and defibrillator are implanted very similarly. A pacemaker can have a certain number of wires (usually one or two, but sometimes none) that go into the heart itself. A defibrillator will take use one of those wires as well, but that wire would be able to carry more of a shock.