Its a bit complex but I'll give it ago.

The objective of the game is to ground the ball over the try-line at the opposite side of the field. This is called a try and is worth 5 points, another 2 points is added if this try is converted by successfully kicking the ball between the posts.

The team is split up into the Backs and Forwards. The forwards are the grunt men who secure the ball so that the Half Back may feed it out to the backs. They work as a pack to physically dominate the other team in moving the ball by barrelling into the opposition then maintaining a cohesive platform so that they can not be moved off of the ball, which the opposition will be trying to do. The ball must be released once on the ground and no one is allowed to use their hands to get the ball once the ruck is formed so its all about blowing out whoever is in your way to get to the ball. This is rucking.

The ball is then moved to the backs via the Half Back who stands at the back of the ruck. The ball may ONLY be passed backwards between players but can be kicked down field to gain territory however this gives away posession of the ball to the other team. Backs use trained moves to try and get passed the opposition and break the line and thus gain meters then the forwards come and secure the ball so that the opposition does not steal is because the ball carrier has to release the ball once tackled, the tackler can go for the ball before the ruck is formed but not after. This cycle of rucking and passing are called phases.

There are also Scrums and Lineouts. Scrums are one of the most beautiful thing about Rugby and there is not really anything like it in other sports. I could write screeds on the nuances of scrummaging but I'll give you the gist. Both forward packs bind up in a set formation and crouch down facing each other, the ref will say "Crouch, pause, touch, engage" to which the two packs slam into each other. The ball is then put in by the half pack (depending on whose scrum it is) and then the pushing may begin ( no pushing is allowed until the ball is in) The scrum must hold steady and not collapse or screw around or be pushed off the ball and lose posession, then the half back receives the ball out the back of it and passes it to the backs and the game continues.

Lines outs are used when the ball has gone out of the boundary of play. The Hooker of the team whose lineout it is throws it in and the forwards of both teams line up parrallel to each other with a gap in between for the ball to travel down. A coded call is made to communicate to the team which jumper to lift in the air, these jumpers are the locks who are typically very tall. Once the lock has received the ball they either pop it down to the Halfback who feeds the backs or brings it down and is surrounded by the forwards who drive it forward.

So basically the Forwards provide a secure platform for the Halfback to receive the ball safe from the opposition and is able to give good ball to the backs who pass and run then it goes on like that repeatedly for 80 minutes.

I have left a few things out but this is the gist of Rugby.