A Stirling engine is a heat engine that is operated by a cyclic compression and expansion of air or other gas (the working fluid) at different temperatures, such that there is a net conversion of heat energy to mechanical work.
The Stirling engine is a closed cycle heat engine. It is typically completely sealed from the outside environment and works on the expansion and compression of the gas (typically air) that's enclosed in the sealed engine. Fuel does not move through the engine like in a conventional internal combustion engine. This means that there is no intake or exhaust.
One side of the engine is heated and the other side is cooled. This causes the gas to go through cycles of expansion and compression. This means it can produce motion by converting heat energy directly into kinetic energy or mechanical work.
There are many types of Stirling engines. There are high pressure engines that are used commercially. There are restored low pressure engines that are used for display. There are table top model engines built by hobbyists and students.
You can find models or kits for almost all of the configurations of Stirling engines including LTD (Low Temperature Differential) engines. An LTD can run on the heat of the palm of your hand.