Answer by Thomas Zerbarini:
The purpose of trim is to "trim off" the excess forces on the flight controls caused by changes in aircraft speed and configuration. Essentially, with a properly trimmed aircraft, the pilot flies the aircraft with small control inputs that are both comfortable and accurate.
Trim works the same on all aircraft big and small. The difference is in the size and complexity of the systems as the aircraft gets larger, bigger and/or goes faster.
Her's the basic trim tab concept for pitch:
Here's is a jackscrew type trimming system used on most large jets. The Mach trim system also utilizes this system for high speed flight:
See thefor more info.
Also look for Mach Trim, Aileron Trim, Rudder Trim.