Pad's aren't defined so much by what they are, but what they do. They're a static sound that holds the chord or root, divorced from any rhythmic content. This allows other instruments to focus on rhythm and melodic content. Lots of things can pads. Alex Lifeson (among many others) does excellent guitar pads using rollbacks and delay. Quiet piano tremolos have been used for padding for hundreds of years (Muzzorgsky, anyone?). Organs (pipe, electric, etc) with only a few stops activated (low harmonics).
But the prototypical "Pad" that everyone thinks of, begins with arco strings, as they can slowly seep in and out of chords smoothly. Synthesizer pads began by emulating strings as closely as possible. This is accomplished by using a sawtooth oscillator, which is very harmonically rich, and then filtering out the high end a bit. Then you lengthen the attack and release quite a lot. Finally, duplicate the oscillator at least one or two more times, and offset their pitch to emulate the rich chorusing of an ensemble. This will get you your basic string pad sound. Of course, you can move away from strings and use other waveforms (square waves sound a bit like a woodwind section of sorts), but keep the idea of filtering out high harmonics, using long attack/release envelopes, and often some kind of chorusing.
Some thoughts. It's really easy to overuse pads, and they're use can be quite controversial. They really play a supporting roll in most cases, and should specifically not get in the way of the other more interesting musical content. Some listeners/musicians hate them altogether because by nature they excuse a more polished symphonic sound, and can be perceived as taking away from raw aggression in hard rock. Conversely, I know some guitarists who LOVE pads because they, "stay out of their way". I'm a pretty busy rhythm and lead keyboardist, and I've had some bandmates go, "could you just play mellotron here?" because, frankly, it insures that the keyboards take a back seat.
Personally, I like using pads in my arrangements, but they're boring as sin to play. So I often play them in my left hand and give my right hand something more interesting to do. I also find myself using them less and less, and try to achieve the same effect with delay lines off my other parts. But they definitely have their place.
