A web server will have a number of pages that it will look for by default if no resource name is included in the URL.
Server-side preprocessed pages will take precedence,
but in absence of these, the server will look for,
in roughly that order of precedence.
Folders that do not have an index.html (or some such) page will, unless there is an htaccess restriction, default to folder view and will list the contents of that directory. At the very least every directory on the server should have an index.html page even if just to say private directory.
If you do not wish the hundreds of images in a folder to be exposed in a directory listing, slip in an index.html page and you’ll ensure a little bit of privacy for that folder.
Above we see a URL with no resource name, but the server will search for an index.html (or similar) in the blog folder so it can respond to the request.
Many hosting plans permit sub-domains up to some number (or no restriction) but the site contents for the sub-domains will all resolve to the same namespace as the main domain. Very likely, their content will be in folders of the same name off the root of the main site.
So long as the sub-domain is registered in hosting plan, this URL and the earlier one above will serve content from the blog directory.
It’s worth noting that
www is a sub-domain, itself.
The site is accessible at the domain,
Recall that only the domain name and TLD are included in the registrar’s records. We have the option of tacking on
www in our marketing material. It’s not a requirement.
Lastly, and what should have been stated off the top, an index page is better known as a, landing page.