Save versus Demo. It's almost entirely for humans and the frontend. Internally they are loaded basically same way, although they have different magic bytes at the start. So loading an hws as an hwd will still work, it'll just show up in Demos instead of Saves in the frontend.

The difference between a save and a demo is that a save is an incomplete game. You can add bytes to terminate a save, or remove them to turn a demo into a save. It can be helpful when debugging a desync/crash.

https://hedgewars.org/kb/EngineProtocol#In-game