Judo and Greco-Roman wrestling are similar in that both are combat sports and use grappling instead of striking. After that, the differences between the sports are greater than the similarities. Although some athletes enjoy success moving from one to the other, everything from history to training methods to scoring works differently.
The History of the Sports
Both judo and Greco-Roman wrestling are modern sports, but they are based on ancient techniques. Judo derives from jiu-jitsu, an assortment of throwing and locking moves that a samurai would use on the battlefied. Jigoro Kano organized those techniques into a teaching curriculum and sport in the late 19th and early 20th century. Greco-Roman wrestling, also organized in the 20th century, was developed to closely match images of wrestlers in ancient Greek and Roman art. Both are still active sports practiced internationally and at the Olympics, as of 2011.
The Difference Between Falls
You can score a "fall" in both Greco-Roman wrestling and judo, ending and winning the match immediately. In judo, this fall is called an "ippon." To score an ippon, you must throw an opponent on the ground so he lies flat on his back, hold him on his back for 25 seconds, or force a submission with a choke or arm lock. In wrestling, only a pin results in a fall. A pin is holding an opponent on his back for three consecutive seconds.
In wrestling, competitors score one to two points for achieving specifically defined positions relative to their opponents. The total score is unlimited, although competition ends if one wrestler establishes a lead of 15 or more points. Judo competition allows partial points for a referee's subjective judgment of how close a participant came to scoring "ippon." Partial points can accumulate, just as in wrestling.
Competitive judo is played wearing quilted martial arts gis. The players can wear either white or blue uniforms, and some tournaments require participants to bring one of each so any given match has contestants wearing opposite colors. Wrestlers compete wearing singlets, stretchy garments that look like an old-fashioned swimming suit. Color choice is unlimited, though most competitors wear a singlet with the colors and logo of their team.