If the ball is caught before it hits the ground, the runners cannot advance from the bases they are on until the moment the catch is made. If they are off the base at that moment, they must return to "tag up", then may proceed to the next base. But, the ball is still live, and the runner has to beat the throw and/or avoid the tag. Note, if the runner is caught waaaaayyyyyy off the his original base, he can be put out at his original base, on a FORCE play (the fielder need only touch the base, with the ball in his possession, before the runner touches it). But if he successfully tags up and is headed for the next base, or back to his original base, the play must be a non-force play (the fielder must "tag" the runner, i.e., touch the runner with his glove, with the ball inside it, before he reaches either base.