A tried is scored when the rugby ball is touched down over the opponents try line, but within the field of play, whilst in sufficient control by the player i.e. it must be intentional.
Originally the try was rewarded with a 'try' at goal, but no points. This origin is still apparent in the award of an attempt at goal ('conversion') following the scoring of a try. The conversion (worth two points in both league and union) has to be taken directly in line with the point at which the try was touched down, but at any distance from the goal decided by the kicker.
Football, before dividing into the forms of rugby and association soccer generally had a similar way of returning the ball into play when it went out of the playing area. The first player to touch the ball when it was out of play would be able to return the ball to the playing area. As the off-side rule meant that all the players had to be on their respective side of the ball and that kicking backwards was almost unheard of, the ball was almost always claimed by a play on the opposing team to the person who had touched it. In this way the touch down in American Football and the try in rugby can be seen as comparable to the corner kick in soccer.