Backs are the players that the forwards support. At each play, the backs receive the ball and pass and run it down the pitch, while the forwards follow the ball and are ready to ruck or maul when the play breaks down.
In general, backs want to remain in a steep line on offense, and a flat line on defense.
Staying in a steep line allows the backs to gain speed before receiving the ball, so they are sprinting full speed whenever the ball is in their hands. Since the ball can only be passed backwards, yardage must be gained by running forwards. Being steep also gives the offense more time to get the ball out to the wing (#11 or #14), who can try to run outside the defense and score.
Here is an outline of how a simple back play might work. Suppose the offense just wants to get the ball out to the wing, as mentioned above. The players first start in the formation shown below. The offense is steep, and the defense is flat. Both the flyhalf (#10) and the weak-side wing (#14) are ready to receive the ball from the scrumhalf (#9). This gives her two options to pass.
Next, the inside center (#12) picks up speed and receives the ball from the flyhalf (#10). Her speed carries her in front of the flyhalf, and now she is leading the line. She looks left to the outside center (#13)..
.. and passes it. Now the outside center (#13) is out in front. This whole time, the defense is moving towards the offense, but staying in a flat line. The scrumhalf (#9) is following the ball, and ready to take control of the situation in case of a breakdown. Not shown are the forwards, who are also following the ball, ready to ruck or maul in case a player gets tackled or drops the ball.
Finally, the ball reaches the strong-side wing (#11). She takes the ball and sprints up the line with it, and takes the tackle, now that the defense has reached the backs. If she can evade the tackle, she might gain a lot of yardage or even score a try, otherwise she will crash into the defense and try to set a clean ruck (preferably) or maul. The forwards take control and try to retain possession of the ball, while the backs reset themselves to do the same thing again, which is referred to as "second-phase play."
To see this in action, here is a short animation of the backline play shown above.
When playing defense, backs must stay in a flat line. As seen below, the red player is running with the ball, into the line of yellow defense. When she reaches the defense, she must either crash into a player and get tackled, or pass the ball off to her teammate. A flat defense can effectively stop even the fastest backs.
When playing defense, you have to remember to keep the entire defensive line flat. If one player breaks from the line and steps forward or hangs back, a hole can be created, allowing the offensive player to slip through the line and break away.
The backs stand in position relative to the current set piece (i.e., lineout or scrum) or breakdown (i.e. ruck or maul). As seen in the diagram and model below, the offensive players (in red) stand steep from the play, while the scrumhalf (#9) waits right behind the play. The defensive players stay in a flat line, and are ready to advance as a line if the red team wins the ball, or to immediately rearrange in a steep line if the yellow players win possession of the ball.
The backline puts its strength on the roomiest side of the field or the "strong side". The other side is logically referred to as the "weak side". In the diagram above, the top of the field is the strong side, and the bottom is the weak side. If the breakdown is on the other side of the field, the flyhalf and centers will change position, but the wings remain on the same side of the field and thus play both weak and strong.