In the game of rugby union, there are 15 players on each team, comprising eight forwards (wearing jerseys numbered 1–8) and seven backs (numbered 9–15). In addition, there may be up to eight replacement players "on the bench", numbered 16–23. Players are not restricted to a single position, although they generally specialise in just one or two that suit their skills and body types. Players that play multiple positions are called "utility players".

Forwards compete for the ball in scrums and line-outs and are generally bigger and stronger than the backs. Props push in the scrums, while the hooker tries to secure the ball for their team by "hooking" it back with their heel. The hooker is also the one who is responsible for throwing the ball in at line-outs, where it is mostly competed for by the locks, who are generally the tallest players on the team. The flankers and number eight are expected to be the first players to arrive at a breakdown and play an important role in securing possession of the ball for their team.

The backs play behind the forwards and are usually more lightly built and faster. Successful backs are skilful at passing and kicking. Full-backs need to be good defenders and kickers, and have the ability to catch a kicked ball. The wingers are usually among the fastest players in a team and score many of the tries. The centres' key attacking roles are to break through the defensive line and link successfully with wingers. The fly-half can be a good kicker and generally directs the back line. The scrum-half retrieves the ball from the forwards and needs a quick and accurate pass to get the ball to the backs (often firstly to the fly-half).

Early names, such as "three-quarters" (for the wings and centres) and "outside-half" (for fly-half) are sometimes used in the Northern Hemisphere, while in New Zealand the fly-half and inside centre are called "first five-eighth" and "second five-eighth" respectively, while the scrum-half is known as the "half-back".

The scrum (a contest used to restart play) must consist of eight players from each team: the "front row" (two props; a loosehead and tighthead, and a hooker), the "second row" (two locks), and a "back row" (two flankers, and a number 8). The players outside the scrum are called "the backs": scrum-half, fly-half, inside centre, outside centre, two wings, and a fullback.