VANOS is a variator system that varies the timing of the valves by moving the position of the camshafts in relation to the drive gear. The relative timing between inlet and exhaust valves is changed.
At lower engine speeds, the position of the camshaft is moved so the valves are opened later, as this improves idling quality and smooth power development. As the engine speed increases, the valves are opened earlier: this enhances torque, reduces fuel consumption and lowers emissions. At high engine speeds, the valves are opened later again, because this allows full power delivery. 
The first-generation single VANOS system adjusts the timing of the intake camshaft in discrete steps (eg the camshaft is advanced/retarded at certain engine speeds. VANOS was first introduced in 1993 on the BMW M50 engine used in the 5 Series.
The second-generation double VANOS system adjusts the timing of the intake and exhaust camshafts with continuously variable adjustment, based on engine speed and throttle opening. The first double VANOS system appeared on the S50B32 engine in 1996.