HOW E-BOMB WORKS.

Anyone who's been through a prolonged power outage knows that it's an extremely trying experience. Within an hour of losing electricity, you develop a healthy appreciation of all the electrical devices you rely on in life. A couple hours later, you start pacing around your house. After a few days without lights, electric heat or TV, your stress level shoots through the roof.
But in the grand scheme of things, that's nothing. If an outage hits an entire city, and there aren't adequate emergency resources, people may die from exposure, companies may suffer huge productivity losses and millions of dollars of food may spoil. If a power outage hit on a much larger scale, it could shut down the electronic networks that keep governments and militaries running. We are utterly dependent on power, and when it's gone, things get very bad, very fast.
An electromagnetic bomb, or e-bomb, is a weapon designed to take advantage of this dependency. But instead of simply cutting off power in an area, an e-bomb would actually destroy most machines that use electricity. Generators would be useless, cars wouldn't run, and there would be no chance of making a phone call. In a matter of seconds, a big enough e-bomb could thrust an entire city back 200 years or cripple a military unit.
The U.S. military has been pursuing the idea of an e-bomb for decades, and many believe it now has such a weapon in its arsenal. On the other end of the scale, terrorist groups could be building low-tech e-bombs to inflict massive damage on the United States.

THE BASIC IDEA.

The basic idea of an e-bomb -- or more broadly, an electromagnetic pulse (EMP) weapon -- is pretty simple. These sorts of weapons are designed to overwhelm electrical circuitry with an intense electromagnetic field.
If you've read How Radio Works or How Electromagnets Work, then you know an electromagnetic field in itself is nothing special. The radio signals that transmit AM, FM, television and cell phone calls are all electromagnetic energy, as is ordinary light, microwaves and x-rays.
For our purposes, the most important thing to understand about electromagnetism is that electric current generates magnetic fields and changing magnetic fields can induce electric current. This page from How Radio Works explains that a simple radio transmitter generates a magnetic field by fluctuating electrical current in a circuit. This magnetic field, in turn, can induce an electrical current in another conductor, such as a radio receiver antenna. If the fluctuating electrical signal represents particular information, the receiver can decode it.
A low intensity radio transmission only induces sufficient electrical current to pass on a signal to a receiver. But if you greatly increased the intensity of the signal (the magnetic field), it would induce a much larger electrical current. A big enough current would fry the semiconductor components in the radio, disintegrating it beyond repair.
Picking up a new radio would be the least of your concerns, of course. The intense fluctuating magnetic field could induce a massive current in just about any other electrically conductive object -- for example phone lines, power lines and even metal pipes. These unintentional antennas would pass the current spike on to any other electrical components down the line (say, a network of computers hooked up to phone lines). A big enough surge could burn out semiconductor devices, melt wiring, fry batteries and even explode transformers.
There are a number of possible ways of generating and "delivering" such a magnetic field. In the next section, we'll look at a few possible EMP weaponry concepts.