Refer to figure.
An aircraft can rotate about three axes. All of these axes pass through the Centre of Gravity (CG) of the aeroplane, and are mutually perpendicular (that is, they are all at 90° to each other).

1. The lateral axis is parallel to the line running through the wings, from wingtip to wingtip. Rotation about this axis is called pitch and this is controlled using the elevator
2. The longitudinal axis runs from the tail to the nose of the aircraft. Rotation about this axis is known as roll which is controlled by the ailerons
3. The Normal axis is vertical; it is perpendicular (normal) to the plane of the other two axes. Rotation around this axis is called yaw and can be controlled using the rudder

The rudder trim tabs are not primarily used to control yaw, in the same way that pitch trim is not used to control pitch directly.