Blocking foul

In basketball, a personal foul is a breach of the rules that concerns illegal personal contact with an opponent. It is the most common type of foul in basketball. A foul out occurs when a player exceeds his or her personal foul limit for a game and is disqualified from participation in the remainder of the game.

Players routinely initiate illegal contact to purposely affect the play, hoping it is seen as too minor to be called a foul. The threshold is subjective and varies among officials and from game to game. Most contact fouls are not regarded as unsportsmanlike. However, a contact foul involving excessive or unjustified contact is classed as an unsportsmanlike foul (or in the NBA, flagrant foul)

Basketball has always had the concept of fouls. In 1891, James Naismith's original 13 rules defined a foul as:

Only the fourth definition remains. Running with the ball and striking it with the fist are now violations. Holding the ball with the arms or body is now rare but legal.

Originally, on a player's second foul, the player would be removed without substitution until the next successful goal (similar to a penalty in ice hockey). Before long, free throws were introduced, originally worth three points each, then one. Originally, any team member was allowed to shoot free throws. In 1924, the rules were changed so that the fouled player shot the free throws.

The victim of a contact foul used to be given three attempts at a free throw, and retained possession of the basketball. Now, a player fouled in the act of shooting gets as many free throws as the point value of the shot he or she attempted (except that, if the shot was successful, the player takes one free throw).

Personal contact does not necessarily constitute a personal foul, unless it gives a player an advantage or puts the opponent at a disadvantage.

In FIBA, the cylinder principle gives each player exclusive rights within an imaginary cylinder defined:

The cylinder extends from the floor to the ceiling, allowing the player to jump upward.

A player can occupy any cylinder not already occupied by the opponent. No one else is allowed to step or reach into this cylinder. A player must not extend his limbs or bend his body in a way that is not normal. If there is a breach of this principle that places the opponent at a disadvantage, the official may penalise it.

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Wikipedia