On a normal engine, the valve timing (the angle of crankshaft rotation during which the valves are held open) is always constant. This kind of timing cannot create the best engine efficiency at both low and high speeds; so it has to be a compromise. If an engine is designed to run best at high speeds (like a race engine); the valves will be held open longer than a normal engine. But this will make the engine run poorly at low speeds, and will produce poor emissions and get low fuel economy. On the other hand, if an engine is designed to run best at low speeds (like a farm tractor) the valves will be held open for shorter periods. This will make the engine idle smoothly, have low emissions, get good fuel economy; but it will have very little power at high speeds. Up until recently, most passenger car engines were designed to run best at medium speeds. They would not run as well as a farm tractor at low speeds, and would not run as well as a race car at high speeds. But as manufacturers became interested in making cars that got the best possible fuel economy, best performance, and lowest emissions; all at the same time, the idea of making engines with valve timing that changed as engine speed increased became very attractive. And that led to the development of variable valve timing.
VVT and VVTI are the names of variable valve timing systems used on many newer vehicles. (VVTI is a more advanced and more effective design than the original VVT). On these engines, the valves are held open for short angles at low speeds, and are opened for longer angles at high speeds. The result is more power at all speeds, along with smoother idling, better fuel economy, and lower emissions. This is done by modifying the design of the valve actuating mechanism. The changes are relatively simple and do not affect reliability. I have driven these engines, and find them much smoother and more responsive than normal engines. I just love this technology.
I hope this helps!!!