Air is constantly, slowly rising and falling in association with temperature-induced changes in density. These temperature variations most often come from dry surfaces like parking lots on the one hand, and cool areas like wet forests on the other hand. On a calm, sunny day without any inversions, the warm columns of rising air (thermals) can be seen with the naked eye: they rise so high that the water vapor starts to condense out of them at a certain temperature, in the form of cumulus clouds.
Thermal soaring involves riding through ('punching') these narrow funnels of air to gain height. Skillfully manipulating the aircraft to punch through as much thermal as possible is one of the most sophisticated parts of manned glider flying. Contests have been created for cross-country glider flying, using these thermals. Typically, it is very difficult to detect a slight change in lift using one's natural sense of inertia, so beeping variometers which identify the climb or fall rate have been created to show the manned glider pilot when he's found one of these invisible rising columns of air, and allow him to circle back, or if the thermal is wide enough, spiral around inside it.
Thermal soaring is a sport among RC glider pilots as well, although a somewhat more difficult one absent any instrumentation. Due to the long spaces between scarce thermals, FPV sailplanes operated at long distances are somewhat more suitable than attempting to follow a plane using an automobile; Nevertheless, due to safety rules this is not uncommon in cross country competitions.
Autonomous thermal soaring is something that has been demonstrated, but the 'free energy' this would allow is highly attractive to UAV enthusiasts.