The Supporter Pack DLC (available for Windows, Mac, and Linux) is a series of media files. It contains behind-the-scenes video clips, pictures, print-quality banner and poster images, and more.
The Supporter Edition of Muffed (and the Xbox Edition) comes with a few goodies in-game. These include a few extra maps for Battle Mode and Survival Mode, comics menu backgrounds, and extra hair types.
The Xbox Edition of Muffed has 3 primary differences. It does not have the Workshop, it has the Supporter Edition in-game goodies, and is limited to 8 maximum players (the Xbox has a maximum controller limit of 8). This Edition also integrates with Xbox features for things like achievements, cloud saves, etc.
The Shareware Demo Editions of Muffed are exactly what they're called: shareware demos. This is a reference to older software and game demos sometimes referred to as "shareware". This term was used to refer to software that users are free to make copies of and hand out to their friends. The demo is available on Steam (which has Steam Edition features as well), but it's much easier to copy the Standard Edition for your friends, since it can run without Steam and without "claiming" the demo on Steam (both of which are required for the Steam features integration).
The Steam Edition(s) of Muffed integrate several Steam features. These include achievements, Steam Input (which allows users to play with Steam Controllers, their phones, and define input assignments via the Steam Configurator), cloud saves, and Steam Remote Play Together (this feature allows you to play Muffed online with your Steam friends).
The Standard Edition(s) of Muffed are how we refer to the base game, free of platform-specific integrations or features.