Aircraft designers can build in a number of devices to help the pilot tweak various aspects of the aerodynamic shape, which can have an impact on control. Trim is an example. Pilots broadly refer to trim when referencing the entire system or the act of using that system, although it is made up of many components.
Every trim system is meant to relieve control pressures. Training airplanes have elevator trim, and sometimes aileron or rudder trim. Higher performance aircraft and all multiengine airplanes also have rudder trim.
The pilot manipulates the trim with a wheel or crank in the cockpit. In the case of elevator trim, that wheel or crank is usually attached to a cable that runs to a pulley inside the tail that turns a jack screw. The jack screw will either change a tab, as is the case for airplanes such as the Cessna 172, or the pitch of the entire horizontal surface in the case of a Piper J-3 Cub and others.
Servo tabs work opposite the direction of the elevator. When the elevator moves up, the tab moves down, and vice versa. This helps alleviate control pressures. Anti-servo tabs move in the same direction as the elevator. They add resistance to the control feel. Moving the elevator down moves the tab down, and moving the elevator up moves the tab up.
