How do elevator servo and anti-servo (geared) tabs differ?
There is some confusion on the different types of tabs that can be found on aircraft: servo tabs, geared tabs, trim tabs.

The servo tab is an aerodynamic lever. It is connected directly to the pilot flying controls. When he moves the controls, he deflects the servo tab only, and feels aeroforces acting upon the servo tab only. When the servo tab deflects, it creates a force moment on the control surface ( elevator, aileron, rudder) which then deflects the control surface until aerodynamic moments are in equilibrium. If there is no airspeed, deflecting the servo tab has no effect on the control surface.

The geared tab changes the flight control forces to either increase them (usually in small aircraft) or reduce them (an arrangement used in the pre-hydraulic past of airliners). The control surface is directly connected to the stick/pedals - deflecting them causes the control surfaces to move. In order to alter control forces, a geared tab moves relative to the control surface: 


For increasing stick forces, in the same direction as the control surface. Wiki and others mention anti-servo tabs, a less confusing name would be positively geared tabs. 
For decreasing stick forces, in opposite direction of the control surface. Whether there is airspeed or not, the geared tab always deflects by a mechanical linkage when the control surface is deflected.


The trim tab has a constant angle relative to the control surface. The angle can be changed by extending/retracting a mechanical link, via a trim wheel or an electric motor with a screw jack.