Custom made orthotics are basically a brace that fits into a shoe. It is designed to align the foot and ankle into the most anatomically efficient position. They look like insoles, but are bio-mechanical medical appliances that are custom made to correct your specific foot imbalance.
Custom orthotics work on your feet much like glasses work on your eyes - they reduce stress and strain on your body by bringing your feet back into proper alignment. The plastic body of the custom orthotic helps to re-align the foot by redirecting and reducing certain motion that takes place during the gait cycle.
Custom orthotics fit into your shoes as comfortably as an insole - and they have the advantage of having been made from precise imprints of your feet.