An engine's valves are what seal the combustion chamber off from the intake and exhaust ports when gasses should be neither pulled in or pushed out, and allow gasses to do so at appropriate times. Without Variable Valve Timing (VVT), valves generally only open and close in a single, predetermined pattern. However, that pattern can only be optimal in a specific RPM range and could limit the potential maximum/peak RPM. VVT allows the valve timing to be adjusted while the engine is running to maximize power output and/or efficency across the full RPM range. Valve timing changes don't alter the timing too drastically; only a small change is required to optimize engine behavior. The change is made by locking a rocker arm into place which is moved by a cam lobe with a different size/shape than the standard lobe.
Variable Valve Timing in Racing Vehicles
The use of VVT is slowly growing due to the steady advances in its reliability, but its benefits are mostly seen in engines which aren't being used in situations that require a lot of shifting. The transition from the first cam setting to the second takes what seems like a small amount of time, but in a racing vehicle, the time it takes for the second setting to engage may be too slow for the engine's rapidly increasing RPM,and it would have to disengage quickly after shifting gears. This can be overcome with a race-focused design (which makes the transition early to overcome the lag), but that would cause the engine to be a poor street car, so vehicles equpped with VVT are rarely driven to the track, and racing them has a higher initial expense.
Other Names For VVT
Variable Valve Timing has been marketed as a number of different things by various manufacturers. For example, Honda's VVT engines have the name V-Tech, while Toyota's method adds "intelligence" to the name, making it VVTi. The concept is the same, but sometimes the method used to change the effect is slightly different.