Saved layouts you can use again.
A template is copied, not connected
When you use a template its content is copied into your page. From that moment they are two separate things.
So editing the template does not update pages built from it, and editing those pages does not flow back. If you want something that changes everywhere at once, that is a region, not a template.
Two kinds: a section or a whole page
A template can be one section — a pricing table repeated across several pages — or a whole page used as the starting point for new ones.
Template scope
A template can belong only to the site it was made on, or to every site on this install. That is chosen when it is saved.
Deleting a template harms no page
Because the content was copied, deleting a template breaks nothing. It only means you can no longer start a new page from it.