A Flat Universe has sufficient mass to slow the expansion down, and bring it nearly to a stop.. but not quite! It can't stop exactly, or it would collapse. Such a universe is always expanding, but it does so almost imperceptively after some time. Such a universe has zero curvature
In a Closed Universe, there is a lot of matter. The gravitational force of all that matter will inevitably slow the expansion down. In this universe, the radius (size) of the universe will reach a maximum at some point, and then the universe will begin to shrink back down until it crashes back into an infinitely small point. Such a universe has positive curvature. Its possible that such a universe might be ``re-born'', bouncing back to make a new big bang. This is called the oscillating universe theory.
