When a player tries to steal the ball from a dribbling player, the referees sometimes award a "reaching foul". I just can't see any difference between a steal and a foul
The specific foul that they're calling (and is often called a "reach in" foul) is the defensive player touching the offensive player ("making contact") in a disallowed way. "Re-routing" as referred in the other answer is actually a different rule - that affects all players, but players guarding the player possessing the ball have a separate, more restrictive ruleset governing them.
Most defensive fouls consist of violations of this rule - basically, don't touch the player with the ball except in specifically allowed ways.
The rule is Rule 12, B, Section 1, (b). Most of Section 1 (except the penalties) is reproduced below.
a. A player shall not hold, push, charge into, impede the progress of an opponent by extending a hand, arm, leg or knee or by bending the body into a position that is not normal. Contact that results in the re-routing of an opponent is a foul which must be called immediately.
b. Contact initiated by the defensive player guarding a player with the ball is not legal. This contact includes, but is not limited to, forearm, hands, or body check.
(1) A defender may apply contact with a forearm to an offensive player with the ball who has his back to the basket below the free throw line extended outside the Lower Defensive Box.
(2) A defender may apply contact with a forearm and/or one hand with a bent elbow to an offensive player in a post-up position with the ball in the Lower Defensive Box.
(3) A defender may apply contact with a forearm to an offensive player with the ball at any time in the Lower Defensive Box.
The forearm in the above exceptions is solely for the purpose of maintaining a defensive position.
(4) A defender may position his leg between the legs of an offensive player in a postup position in the Lower Defensive Box for the purpose of maintaining defensive position. If his foot leaves the floor in an attempt to dislodge his opponent, it is a foul immediately.
(5) Incidental contact with the hand against an offensive player shall be ignored if it does not affect the player’s speed, quickness, balance and/or rhythm.
c. Any player whose actions against an opponent cause illegal contact with yet another opponent has committed the personal foul. d. A personal foul committed by the offensive team during a throw-in shall be an offensive foul, regardless of whether the ball has been released. e. Contact which occurs on the hand of the offensive player, while that hand is in contact with the ball, is legal.