Andy is correct. The traditional VTEC basically uses an oil pressure-actuated pin to lock rockers together and follow a single cam lobe. The net effect is basically a 'switching' of cams at a specific RPM. You get a good low-RPM cam at low revs, and a good high-RPM cam to keep the torque alive at higher RPMs. (Note: not *all* VTEC cars worked exactly this way. VTEC has also been used in various incarnations, both two and three stage, to improve milage by shutting valves OFF)

The strength of VTEC is that you essentially get two cams in one, with variable lift and duration. There wasn't really another system on the market that accomplished that, though some other manufacturers do now.

The weakness is that you get ONLY two (or arguably three with a 3-stage VTEC). It's either on, or it's not. There's no variability.

You can think of it like a transmission. A one speed tranny would suck for obvious reasons. Two speeds is better, but still not great. Five speeds are good. But the ultimate is an infinite number of gears (like some manufacturers, including Honda, are trying to do now with CVT).

iVTEC is the same thing, but with variable cam timing thrown into the mix. Now the engine can not only use one of two cam profiles, but it can also infinitely vary the timing via an electronically-controlled cam gear. With iVTEC, the engine can vary the lift and duration (between only two profiles), as well as the timing of the intake and exhaust cams with respect to one another (on a DOHC, doubt Honda would ever bother with variable cam timing on a SOHC, doesn't make much sense from a cost/benefit standpoint).

The Holy Grail is infinitely variable duration and lift, along with infinitely variable timing. This is all but impossible using a traditional mechanical camshaft. The valves would have to be independently, electrically controlled. You'd better believe manufacturers are looking for a way to do this, but right now it's just not practically possible.