What are the offside rules?

Attackers: an attacking player in a position forward of the ball is offside. A tap cannot be taken until all attacking players are onside.

Defenders:

• At a rollball, all defenders must retire seven metres, or to the defending try line as the referee indicates. The penalty goes to the attacking team at the point of the infringement or on the seven metre line directly forward of it.
• At a tap, defenders must retire ten metres from the mark, or to the defending try line as the referee indicates.
• At a rollball or tap, defenders must not retire an unreasonable distance beyond their try line.
• After making a touch, the defender must retire the required seven metres (or to the try line) without interfering with the attack. The penalty is ten metres forward of the infringement.
• Defenders must not move forward of the onside position until the half has made contact with the ball, unless the referee directs it. If the half is not within one metre of the rollball, onside defenders may move forward as soon as the player performing the rollball releases the ball.
• A tap cannot be taken until at least four defending players are onside, unless the referee directs otherwise.

If an attacker intentionally touches an offside defender who is making every effort to retire and stay out of play, the touch counts.

Source: FIT Playing Rules 5th Edition, definitions and rules 7.5, 7.6, 10.10, 13.12 and 15.