When you save a deck with a ".deck" extension, Decker will write out a file that can be opened in Decker.
When you save a deck with a ".html" extension, Decker will package the deck as a single HTML file containing a JavaScript payload with everything needed to run (and edit) the deck in a web browser. You can share this HTML file with friends directly, put it on your own website, host it on an itch page, etc.
You can also save a "protected" version of a deck via "File -> Properties... -> Protect...", which really just saves a copy of the deck with the "locked" property set to 1, causing it to hide the menu bar and editing tools; this might be handy for publishing something like a game.
And just to be clear, all of these features work the same whether you're saving a document created from the native version of Decker _or_ the web-based version of Decker.