In Law 12.1, the following definitions are provided for careless, reckless, and excessive force:
Careless is when a player shows a lack of attention or consideration when making a challenge or acts without precaution. No disciplinary sanction is needed.
Examples of careless fouls are shown here, here and here.
Reckless is when a player acts with disregard to the danger to, or consequences for, an opponent and must be cautioned

A player who commits a reckless foul is cautioned for unsporting behaviour.
In Law 12.3:
There are different circumstances when a player must be cautioned for unsporting behaviour including if a player:
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- commits in a reckless manner a direct free kick offence
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Examples of reckless fouls are shown here, here1 and here.
1 - The referee appears to give the decision to the incorrect team here.
Using excessive force is when a player exceeds the necessary use of force and endangers the safety of an opponent and must be sent off

A player who commits an excessively forceful foul is sent-off for:
- serious foul play, if the foul was committed whilst challenging for the ball; or
- violent conduct, the foul was committed while not challenging for the ball.
In Law 12.3:
A tackle or challenge that endangers the safety of an opponent or uses excessive force or brutality must be sanctioned as serious foul play.
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Violent conduct is when a player uses or attempts to use excessive force or brutality against an opponent when not challenging for the ball, or against a team-mate, team official, match official, spectator or any other person, regardless of whether contact is made.
Examples of excessively forceful fouls are shown here, here and here.