's explanation is about right as far as I know. Rugby was fairly popular in the US for much of the late 19th century, but over time some people, particularly teams at certain East Coast colleges, began to introduce variations on the game. The most comprehensive of these changes were those brought about by Walter Camp at Yale, who came up with concepts pretty foreign to any Rugby player such as the quarterback, the first down, and the forward pass. These changes fundamentally changed the game in a few key ways: the down/scrimmage system meant that play stopped much more frequently; the forward pass meant that kicking was no longer as useful; and blocking ahead of the runner in combination with relaxed tackling rules made the game much more physically aggressive - dangerously so in the early days. However, to the players and fans involved, it was still more or less the same game, whose name they had shortened from "Rugby Football" to just "Football", until the new rules were codified and it came to be accepted as a distinct sport. By then the name was ingrained and it stuck.