There are two types of EMP bombs, nuclear and non-nuclear.
The nuclear explosion generates ‘ionizing radiation’. Ionizing radiation is radiation with enough energy so that during an interaction with an atom, it can remove tightly bound electrons from the orbit of an atom. It is the instantaneous explosive pulse of these spinning dislodged electrons which creates the Electromagnetic energy that causes the damage from the EMP effect.
The result is a a huge electrical current that is generated in an extremely short pulse of time. This huge current will pulse through a coil, which in turn creates a massive pulse of Electromagnetic energy that causes the EMP damage.
EMP Circuit Effect
The pulse of Electromagnetic energy (regardless of what produced it) induces currents in circuits (just think of circuits as anything with ‘electronics’ inside). Depending on the amount of Electromagnetic radiation, the current that is induced in these circuits can be huge or small. If the current is large enough, it will burn out a circuit due to internal resistive heating.
Circuits are everywhere, and they are in most every ‘thing’ that we use today. Anything that has a connected loop of wire is a circuit (including the power grid power lines and grid transformers).
EMP does not affect living organisms because we (and other living organisms) are not conductive in a way that will cause damage… we do not have a connected loop of wire in us. However if a person depends on a circuit to live (pace-maker, life support) then that would obviously be a bad thing.
It is important to remember that a circuit can be protected by building a Faraday cage around it. A Faraday cage is a conductive casing that prevents the Electromagnetic radiation from reaching the circuit. This type of protection is likely nullified if there is anything connecting the inside of the cage to the outside of the cage (power cords, circuit touching cage, etc.).
An electromagnetic pulse (EMP), in effect, destroys the junctions of the transistors in electronic circuits. It is a high voltage, high frequency, high energy pulse. It has little or no effect on living tissue, due to its extremely short duration. Having said that, it WILL have a potentially terrible effect on humans and human civilization due to the dependencies that most of us have on electronic infrastructure to keep us fed and alive.
There has apparently been considerable and renewed interest in EMP Weapons today by many nations. Unfortunately, indirect consequences of using such weapons will be just as, or more devastating than other weapons of mass destruction given the reliance that developed nations have on electronic infrastructure.