A caution. Two yellows = red card and you're off. Given for: reckless fouls, time-wasting, dissent, removing your shirt to celebrate (yes, really), and "unsporting behavior" (a catch-all for anything the ref doesn't like).
Straight red = serious foul play, violent conduct, denying a goal-scoring opportunity, or offensive language. You're off immediately and banned for at least 1 game. The walk of shame to the tunnel is football's most humiliating moment.
VAR only intervenes for 4 things: goals, penalties, red cards, and mistaken identity. The on-field ref makes the final decision after viewing the pitchside monitor. VAR cannot check yellow cards, throw-ins, or corners. The "clear and obvious error" threshold means many controversial decisions stand.
You're offside if any part of your body that can score a goal (not arms/hands) is beyond the second-to-last defender when the ball is played. Semi-automated offside technology now draws the lines. Armpit offsides are real and infuriating.
The referee adds time for stoppages: substitutions, injuries, VAR checks, time-wasting, and goal celebrations. Since the 2022 World Cup, added time has increased dramatically — 10+ minutes is now common. Fergie Time was ahead of its time.