In playing Rubgy Union, you'll find that depending on where you play, you'll be expected to perform very different tasks. To reflect this, you'll notice that player's numbers are locked to their position. Here's a really quick breakdown of the positions, just in case you're interested.
There are eight Forwards (numbered 1-8). Their chief responsibility is to gain and retain possession of the ball, whether in open play or from set pieces such as the scrum and the line-out. They will generally contain the most Rueben-esque members of the team, using their size and stature to scare the hell out of anyone unfortunate enough to be in their path.
The loosehead prop, erm, props up the hooker (who'll be reaching out for the ball with his legs) in the scrum and holds the jumpers in the line-out. So called because they are not locked into the scrum.
Identical to the loosehead but named 'tighthead' because they are locked between two opponents (the loosehead prop and the hooker).
Hookers are unsurprisingly responsible chiefly for hooking the ball with their feet in the scrum. They also usually throw the ball at line-outs and make weird calls that no-one really understands.
The locks are typically the tallest players and act as targets at line-outs, having to catch and distribute to the scrum-half or at the very least wildy flap about and knock the ball toward their team's side. At the scrum, they are vitally located between the props and the hooker, and provide balance and, if they're pushing hard enough, a bit of momentum.
The flanker is a curiosity in rugby union, being the only true all-rounder position with no set duties. In the scrum, they are not big pushers and, although they must stay locked to the scrum until the ball is out, they must respond quickly and unbind when it does.
Pretty much identical to the blindside flanker, the openside equivalent is usually a bit smaller but a lot faster.
The number eight is the link between the the Forwards and the Backs, and is a bit of a both. As well as having the least exotic of all the position names, their fundamental role is to augment the team’s ball-winning and ball-carrying.
There are seven Backs (numbered 9-15) who are expected to both create and convert point-scoring opportunities after the ball has been won and taken on by the Forwards. They have to be agile and dynamic - because it's doubtful the forwards will be! Kicking skills are also a must in certain positions.
Much like the number eight links the Forwards to the Backs, so too the scrum-half connects the Backs to the Forwards.
The scrum-half is involved in play at all times - they'll pop the ball into the scrum and hopefully collect it after it is passed back. They will also be the player standing at the side of the lineout waiting to receive the ball from the jumpers.
Arguably the most influential player on the pitch, as he calls the tactical game on the pitch with his kicking, distribution and ball-carrying from deep. The fly-half is also typically the goal-kicker. As a result, the fly-half has to be a leader, a powerful and accurate kicker, and yet also perform to the highest standard in defense.
The wingers are big try scorers and are usually seen on hand finishing a successful attack. They should be the quickest players and able to run rings around the opposition.
Like their flanker counterparts in the Forwards, the Centres are all-rounders, but with extra power, mobility and handling to augment the defensive capabilities of the Backs as well as provide some venom to the offence. Both Centres receive frequent balls from the fly-half to test the opposition’s defensive line and expose gaps which either they or the wingers can profit from.
The fullback acts as the last line of defence. They are often required to catch long kicks from the opposition and enact counter attacks with the newly-acquired possession.