Normally if any fielder is in the way of the runner, it is obstruction.
However it is not obstruction if the fielder is fielding a batted ball or has possession of the ball.
If a runner hinders a fielder fielding a batted ball, it is interference.
If there is obstruction, the runner is awarded the base they would have obtained had there been no obstruction.
If there is interence, the runner is out.