When I run jspm install Aurelia-framework at the command prompt, I see a message Updating Registry Cache....
Is the jspm updating the Windows Operating System registry or package.json file in the project or something else?
It means that jspm-cli fetches https://github.com/jspm/registry which provides overrides and short names for popular packages. See the source code to understand what's happening when jspm updates the registry cache: https://github.com/jspm/registry-endpoint/blob/master/registry.js#L182