A clone is where you have proper duplication, and separation between, two (possibly different) versions of a repository. When one repo is amended, the new content must be actively copied to the other repo using a push command. And changes in the other repo fetched.
When you fork a repo, on a server, there is no need for duplication of content because both repos will use the same [fixed object] content from that same server. The 'trick' is in managing the different user viewpoints so that each user believes they have a full personal copy of the repo. Pushes and fetches between forks is simply updates the user's pointers.
At a lower level, git does the same thing internally. If you have three different files, each containing Hello World, then git simply 'forks' its single copy of the Hello World blob and offers it up in each of the three places as required.
The ability to fork on the server means that Github's large storage allowance isn't that big on average as every body shares the one single underlying repo.