What do yellow and red cards mean in rugby union?

• Yellow: the player is "cautioned and suspended for 10 minutes".
• Two yellows: a player who commits another yellow-card offence "must be sent off".
• Red: the player "will take no further part in the match". "A player sent off may not be replaced."
• Elite rugby only: "For intentional and highly dangerous actions, the player cannot be replaced. For other actions, the player sent off may be replaced after 20 minutes playing time."
• Dangerous tackles: early, late or dangerous, including any that leads to head contact or is above the sanction line.
• Other dangerous play: tackling a player without the ball or in the air, and lifting and dropping a player on their head or upper body.
• At rucks and mauls: charging in without binding, contact above the shoulders and collapsing.
• Obstruction: blocking an opponent from the ball or the ball-carrier.
• Repeated offences: after a general caution to the team, the next player to repeat the offence is shown a yellow card.
• Misconduct: disputing decisions and anything against the spirit of good sportsmanship.
• Penalty try: a player whose foul play prevents a probable try must be shown a yellow or red card.
• A front-row player carded: another player may have to leave so that a front-row replacement can come on.

The glossary has the yellow card, the red card, the sin-bin, the foul play entry, the high tackle and the obstruction entry.

Ten minutes is an eighth of the match with a team one short, which usually costs points. The 20-minute red card in elite rugby punishes the player for the rest of the match while sparing the team from playing a long time short for an action that was not intended. The worst actions still leave the team short for good.