[masraum]

I've found several different things online that seem to indicate something similar to the below.

Let's set aside motorized gliders for a moment, and just consider un-powered aircraft. All sailplanes are gliders, but not all gliders are sailplanes.

A glider is any airplane without an engine.

A sailplane is a glider that is aerodynamic enough that sustained flight is possible by using rising air currents to maintain altitude, or even to climb.

A glider that is not a sailplane is not expected to maintain altitude or travel very far. During world War II the US, Britain, and Germany all used large troop carrying gliders. These could in no way be called sailplanes.

All gliders need to be provided some form of propulsion to get off the ground - often they are pulled by an airplane, or pulled by a wench on the ground. To allow them to fly launch without assistance, powered gliders have been developed, using a very small engine to take off. This is turned off and the propeller folded back to allow it to operate as a sailplane.