link-symbolWhen you are using the link button (it looks like three little chain links) on a text editor for creating an
<a href="/"> tag to link to another page on your site, a dialog box will appear with a text box to insert the URL.

TinyMCE

If you are using the default TinyMCE default editor, there is just the text box, where you can type in a URL or use cut and paste. If you want to put in the URL of another page on your site, you can open up that page in a browser and copy and past the URL from the address bar then paste it into the text box next to Link URL. You can also go to that Menu Item in menus and get the Joomla PHP generated code for that page in the Details tab next to link. Yo can click in the box three times rapidly and it highlights the entire line.  Use CNTL + C to copy the text.

JCE Editor

If you have installed the JCE Editor, as I recommend, when you use the link button to create a link, it brings up a more elaborate dialog box for entering in the URL for the link.  

You can choose an article from the Content section - which means you can link directly to an article even if it is not used by a Menu Item. You can also choose from a Menu Item and get the PHP generated code for that Menu Item. The nice thing about using the JCE Editor link generator, it it then generates a friendly URL for that link rather than putting in the PHP gobblety-gook code in the address bar.

If you use the PHP generated code for the href, it will always generate that page, even if you rename the Menu item.  This will not be the case if you cut and paste the code from the address bar on the browser page.

Note: If you are linking to a page on your website and you choose to copy and past the URL from the browser window, you can delete the http:// and the domain name, and index.php if it is present.