What are cautionable offenses?

Answer 1

A yellow card is shown to a player or substitute to signal they are being cautioned. A caution is given for misconduct. Misconduct may, or may not be, a foul. For example, it would not be a foul to delay the restart of play but it would be misconduct.

A player or substitute is cautioned if they commit any of these:

1. unsporting behavior
2. dissent by word or action
3. persistent infringement of the Laws of the Game
4. delaying the restart of play
5. failure to respect the required distance when play is restarted with a corner kick, free kick, or throw-in
6. entering or re-entering the field of play without the referee's permission
7. deliberately leaving the field of play without the referee's permission

Under the FIFA Laws of the Game a cautioned player may continue to play. Under NFHS (US High School rules) a cautioned player must leave the field of play and may be substituted for.
The referee shows a player a yellow card for a first offence in football. A second offence in the same match will result in being shown a second yellow card, and immediately shown the red card and sent off the field. There are various grades of offences, so the referee has to make a judgement.