A try is a way of scoring points in rugby union and rugby league football. A try is scored by grounding the ball (the ball must be touching the player when coming into contact with the ground) in the opposition's in-goal area (on or behind the goal line). Rugby union and league differ slightly in defining 'grounding the ball' and the 'in-goal' area.

The term try comes from try at goal, signifying that originally, grounding the ball only gave the opportunity to try to score with a kick at goal.

A try is analogous to a touchdown in American and Canadian football, with the major difference being that a try requires the ball be simultaneously touching the ground and an attacking player, whereas a touchdown merely requires that the player in possession of the ball enter the end zone. In both codes of rugby, the term touch down formally refers only to grounding the ball by the defensive team in their in-goal.

More Info: en.wikipedia.org