If the "real thing" is a duel with sharps, then aside from the mortal danger and related psychological factors, the primary technical difference is that the duelist can win with only a single good touch, whereas the athlete has to hit his opponent as many as 15 times and so requires more technical and tactical depth. Many inferior duelists have won their combats through sheer dumb luck. This is far less likely in the sport. On the other hand, the sport fencer takes many defensive risks that would be unthinkable in a duel, since he has up to 15 "lives" to work with. Some purists equate "real" fencing with classical fencing, ie. the prevalent styles of the traditional French and Italian schools of fencing that predominated before electric fencing was popularized. By comparison, modern fencing is more mobile and athletic, while classical fencers were known for their more sophisticated phrasing and bladework.

Modern sabre fencing is performed with lightweight weapons and techniques that do not translate well to military sabres and broadswords. There is a certain amount of cross-over with lighter turn-of-the-century dueling sabres, however.

Lastly, it just seems apparent to some that sport fencing has evolved away from its bloody origins. Tactically and psychologically, it is true that the sport is a vastly different world from the duel. The sport fencer's life is never in jeapordy, and with as many as 15 hits needed to secure victory, there often isn't even much figurative danger. Since the quality of a hit is immaterial, fencers will naturally prefer an easy "wounding" hit over a difficult "fatal" one, and so glancing hits will often win out over strong thrusts. Technically, however, there have been few modern innovations, and the sport fencer still possesses all the technical skills necessary to fight a duel.

Quoted from the Offical Fencing FAQ

Also see:

• Historical Overview of Modern Fencing - Includes information on right-of-way and Olympic Rules
• History of Fencing - Another good historical overview from the FAQ