It's the same as the difference between a sailplane and a glider. See: What is the difference between a glider and a sailplane?
(By the way, that question that I linked ought to be reopened. I just edited and cast a vote to reopen it.)
In particular, a glider is any aircraft designed primarily for unpowered flight. A sailplane, on the other hand, is a glider with conventional control surfaces (ailerons, elevator, and rudder—as opposed to, say, a paraglider or a hang glider) which is designed for staying aloft in updrafts (as opposed to a cargo glider, which is intended to simply travel a reasonable distance and then land).
Although the concepts are different, I'm not sure both tags need to exist. Perhaps we could get by with just the glider tag and do away with sailplane.