Basketball is sometimes called a non-contact sport. Although, there is plenty of legal contact between players, some contact is considered illegal. If an official decides that the contact is illegal, they will call a personal foul.
Most of the fouls in a game are committed by the defense, but the offense can commit fouls as well. Here are list of some of the types of fouls.
Typical Defensive Fouls
Blocking - A blocking foul is called when one player uses their body to prevent the movement of another player. This is often called when the defensive player is trying to draw a charge, but does not have their feet set or initiates the contact.
Referee signal for blocking foul
Hand Check - A hand check foul is called when a player uses their hands to impede or slow the movement of another player. This is usually called on the defensive player covering the player with the ball on the perimeter.
Holding - Similar to a hand check foul, but is generally called when a player grabs another player and holds on to prevent them from moving.
Illegal Hand Use - This foul is called for any use of the hands on another player that the referee thinks is illegal. It's generally called when you hit another player on the arm during shooting or when trying to steal the ball.
Typical Offensive Fouls
Charging - Charging is called on the player with the ball when they run into a player that already has position. If the defensive player doesn't have position or is moving, then generally the official will call blocking on the defender.
Referee signal for a charging foul
Moving Screen - A moving screen is called when the player setting the pick or screen is moving. When setting a screen you have to stand still and maintain position. Sliding a bit over to block your opponent will cause a moving screen foul to be called.
Over the Back - This foul is called when rebounding. If one player has position, the other player is not allowed to jump up over their back to try and get the ball. This is called on both offensive and defensive players.
Who Decides?
The officials decide if a foul is committed. While some fouls are obvious, others are more difficult to determine. The referee has the final say, however, arguing will get you nowhere.
Sometimes referees will call the game "close". This means they are calling fouls with just a little bit of contact. Other times the referees will call the game "loose" or allow more contact. As a player or coach you should try to understand how the referee is calling the game and adjust your play accordingly.
There are various penalties for fouls depending on the type of foul. You can read more about it on the basketball penalties for fouls page.
