While Snippets are meant to be used within a single template, Shared Content is created with multiple templates in mind and can be used in any number of different templates.
Like Snippets, the Shared Content Library is a repository of macros, functions, HTML, and other coded content for use within templates. Since Shared Content is managed as an asset, each can be organized into folders and have differing Permissions applied.
The code for including Shared Content in a template looks like this:
Add Shared Content
To add new Shared Content, navigate to Content > Shared Content, click the Actions drop-down menu, and select "Add Shared Content". Provide a name for the Shared Content, as well as a description (if desired), then click "Save".
After saving, you'll see the Shared Content edit screen. Here you can create or edit your content, import a file, and test the shared content against a variety of different templates.
Once a piece of Shared Content is created, it will be available to reference in any template within your organization.