Obstruction and Interference
Obstruction is the fielder in the runner's way. Interference is the runner or batter in the fielder's way.
How it actually works. They are mirror images and the umpire's remedy differs. Obstruction protects the runner: he is awarded the base the umpire judges he would have reached, and on the type where a play is being made on him the ball is dead immediately. Interference protects the defense: the offender is out and, in most versions, the ball is dead and other runners return. Catcher's interference is a third thing again, where the catcher's mitt touches the bat, and it gives the batter first base.
Why it is done, and when it is wrong. The distinction that trips people up is the ball, not the contact. A fielder standing in the baseline without the ball is obstructing even if the runner ran into him, and a fielder who has the ball or is in the act of fielding it owns his ground and the runner has to go around. That is why you see a third baseman step off the bag when a throw is not coming: standing there without the ball is the violation.
At Little League and travel ball. Obstruction is called far less than it happens in youth ball, usually at first base and at home plate, and the reason is that youth fielders straddle bases they have been taught to straddle. It is worth teaching a young infielder to give the runner the front edge of the bag, because it is both legal and safer.
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