To make links you will use the "a" tag, this will take the user to the next page. If you are linking another page you will need to the web address or URL this will appear blue with a underline. IF you are linking pages of the same website address than you will not need to include the URL.
A directory structure is essentially a pathway of folders enclosed in the root folder. The main homepage of site is called the index. Editing a template file would change all the pages that use that template to not change the HTML or you will break the page.
Using relative URL's are used to link pages in your own website which makes the process quicker and easier. If all the files for your site are in one folder you should use that file name for your page.If a website is live a few techniques will not work if your filea are stored locally.