What is the difference between rugby and American football?

The games share an ancestor. American football grew out of rugby football played at American colleges in the 1870s, and it became a distinct sport largely through the work of Walter Camp at Yale, who introduced the line of scrimmage in 1880 and the system of downs in 1882 to fix a game that had become a stalemate of teams simply holding the ball. Those two changes created everything distinctive about American football: planned plays, a strategic pause between them, and possession measured in yards gained.

The visible differences follow from that. American football permits a forward pass, which rugby forbids entirely, since a rugby ball may only be passed backward or sideways. American football allows blocking, meaning teammates may physically obstruct opponents away from the ball, which is illegal obstruction in rugby. Play stops after every tackle in American football and continues in rugby until a genuine stoppage, so a rugby match runs two 40-minute halves of nearly continuous action while an American football game spreads about 11 minutes of live play across three hours. Unlimited substitution in American football produced extreme specialization, with separate offensive and defensive squads, while rugby players play both ways with limited replacements. The heavy protective equipment is downstream of the blocking and the higher-speed collisions it permits.