What is the difference between a glider and a sailplane?

The term "glider" has many usages. It can mean any unpowered object, from lizards to underwater craft, supported by wings and able to convert part of its downward motion into significant forward motion. Most commonly it describes any unpowered fixed-wing aircraft, some of which have quite steep and short flight paths.

The more streamlined gliders can soar, which is a form of gliding in which rising air currents are used to sustain flight more or less indefinitely. Such a soaring glider is commonly called a sailplane.

Then there are the specialist technical definitions, such as that used by the American FAA certification authority.

Many glider clubs exist and most of their planes are of the sailplane class of glider, however they are typically referred to as gliders by their owners and pilots.

"Glider" is the broader term, including everything that would normally be described as a "sailplane", and also including many other aircraft such as troop-carrying assault gliders, and ultra-low-performance training gliders like "primary" trainers and training gliders adapted from light airplanes such as the Piper TG-8, and rocket-powered gliders such as the ME-163 Komet, and even the Space Shuttle.

"Sailplane" generally confers a connotation that a given aircraft is intended to actually gain altitude in soaring flight. Yet there are also some craft called "gliders" that are specifically intended for soaring, yet would never be called "sailplanes". These include weight-shift controlled hang gliders, and paragliders. As an aside, being ultralight aircraft, note that these aircraft do not actually fit within the glider "category" as defined by the FAA.

If a given aircraft is not generally shaped like a streamlined, efficient version of a conventional airplane, or is not guided by a pilot using a fairly "conventional" aircraft control system, it will probably not be called a "sailplane", no matter how well it can soar.

There are also some aircraft that most people would agree may fairly be called "sailplanes" as well as "gliders", yet would not fit within the "glider" category as defined by the FAA because they are ultralight aircraft. An example would be the Archaeopteryx foot-launchable sailplane.

From the FAA Glider Flying Handbook, pages 1-3 and 1-4:

The Federal Aviation Administration (FAA) defines a glider as a heavier-than-air aircraft that is supported in flight by the dynamic reaction of the air against its lifting surfaces, and whose free flight does not depend principally on an engine. ...

Another widely accepted term in the industry is sailplane. A sailplane is a glider (...) designed to fly efficiently and gain altitude solely from natural forces, such as thermals and ridge waves. Older gliders and those used by the military were not generally designed to gain altitude in lifting conditions.

(Emphasis mine)

I have no issue with either answer given. That said, I think this question deserves an answer "from the street".

I got my glider add-on in the summer of 2016, started training for it in the summer of 2015. I belong to a 100+ member glider club in the United States, of which 85 percent have their glider certification/add-on, 20 percent are CFIs and quite a few have flown gliders for 40+ years. Due to the nature of our club, during my training I flew with 15+ different instructors. And, of course, given the "stand around and tell war stories" nature of glider operations, I've talked glider flying with most of my fellow club members. During all flights I have undertaken and observed, the primary goal was ALWAYS to "remain airborne by taking advantage of thermal currents." (though we don't call them "currents"). The exception, of course, is training flights, where the primary objective was learning to fly gliders.

All of this said, I have NEVER, not once, heard the aircraft we fly refered to as a "sailplane". On the contrary, the word we use to refer to what we fly is glider, exclusively.

There does not appear to be any formal definition I can find, but if one observes the aircraft that are typically described as a 'glider' or a 'sailplane'...

Sailplane is generally used to describe an unpowered aircraft that is optimized to remain airborne by taking advantage of thermal currents. Typically lightweight, typically with very long wings. Schweitzer is a well known maker of sport sailplanes.

Glider refers to an unpowered aircraft that isn't necessarily optimized to remain airborne in the non-towed state. Aircraft described as 'gliders' are usually WW2 aircraft for getting troops and equipment onto unprepared ground, hopefully in one piece. They were used for airborne assault in pre-helicopter days. Examples: Waco CG4, Airspeed Horsa, DFS-230, ME-321. (the ME323 Gigant was the powered version) All were expressly designed to be towed to their destination by powered aircraft, and make short glides onto unprepared ground, to deliver troops and equipment.

Also, rocket propelled aircraft such as the ME-163 were described as a glider, when their fuel ran out. Not sailplane.

Finally, quite a few aircraft under development in the pre-CAD era were first flown as unpowered versions. Those experimental aircraft are always referred to as 'gliders'.

The sailplane article is focused around sport and recreational planes. The aircraft article describes a wide variety of aircraft, not simply planes, like hang gliding and paragliding.

Which makes sense - one is about aircraft, so it has the aircraft moniker. The other is specifically about planes, which are a subset of aircraft, and thus have the sailplane moniker.

There was an idea of merging them in 2011, but it failed.