Under vertical load, bridge and building beams are subject to tensile and compressive stresses of varying intensities. The top half of the beam is subject to compression, and the bottom half is subject to tension.
Concrete is strong in resisting compression, but weak when subjected to tension. Thus, steel reinforcement is used to help concrete beams resist or compensate for tension where needed. To reinforce a concrete beam, prestressing is applied to the area of the beam which will undergo tensile stress. Since concrete is strong when compressed, prestressing takes advantage of this trait and compresses the necessary areas of the beam to balance out the tension which will occur under load. When the load is applied, the areas which were compressed by prestressing are able to stretch the necessary amount without loss of strength.
Pre-stressing can be applied in two different procedures: pretension and post-tension.
Pretension is the process whereby prestress steel strands are tensioned and then concrete is cast around them. After the concrete has hardened, the steel is released which in turn compresses the concrete.
Post-tension is a process which begins with a duct placed into the beam form. Steel strands are then threaded into the duct and post-tension anchor blocks are attached to the duct at both ends. Concrete is then cast into the form around the duct assembly. After the concrete has hardened, the strands are tensioned by jacking against the anchor blocks, inducing compression where necessary.
This definition is courtesy of my father, who has over 40 years experience as a civil engineer, specializing in bridge engineering.
Note: Most of the bridges mentioned in the softlinks below were not made with prestressed concrete as prestressed concrete cannot span such large distances. The Chesapeake Bay Bridge Tunnel was, as are most of the concrete highway bridges one sees where a stretch of highway crosses another.