A play that is fairly common is either a baserunner runs into a fielder making a play or a fielder gets into the way of a runner. Neither may be intentional but how should they be handled?
The offensive player getting in the way of a fielder making a play is interference. A fielder (not making a play) getting in the way of a runner is obstruction.
Interference : League Rulebook Section 2 (definition of terms) defines offensive interference as an act which obstructs, impedes, hinders or confuses any fielder attempting to make a play. If the umpire calls the runner out for interference, the ball is dead and all other runners return to the last base legally touched at the time of the interference. The interference does not have to be intentional. Rule 7.09(j) - (Interference) also states that "it is interference by a batter or a runner when the runner fails to avoid a fielder who is attempting to field a batted ball."
Before everyone asks about inadvertent contact on thrown balls, Rule 7.08(b) says that a runner is out when he "intentionally interferes with a thrown ball.”
So - long story short, any runner who obstructs, impedes, hinders or confuses any fielder attempting to make a play on a ball is out whether intentional or not. Any runner who interferes on a throw is out only if intentional.
Obstruction: League Rulebook Section 2 (definition of terms) defines obstruction as "the act of a fielder who, while not in possession of the ball, impedes the progress of any runner. A fake tag is considered obstruction." The definition also states "NOTE: Obstruction shall be called on a defensive player who blocks off a base, base line or home plate from a base runner while not in possession of the ball."
Rule 7.06 (The runner) states that when obstruction occurs, if a play is being made on the obstructed runner he shall get at least one base beyond the base last legally touched and all other runners shall advance to the base they would have reached if there had been no obstruction. If no play is being made on the obstructed runner, the play should continue and afterwards, the umpire shall impose any penalties that will nullify the obstruction.
So for obstruction, the fielder does not have to intentionally block the baseline of the runner. The fielder just needs to impede the progress of the runner.