Git fork is git clone?

Fork, in the GitHub context, doesn't extend Git.
It only allows clone on the server side.

When you are cloning a GitHub repo on your local workstation, you cannot contribute back to the upstream repo unless you are explicitly declared as "contributor".
So that clone (to your local workstation) isn't a "fork". It is just a clone.

The other solution to contribute to that GitHub project is to:

• clone that GitHub repo on your GitHub account (that is the "fork" part, a clone on the server side)
• contribute commits to that GitHub repo (it is in your own GitHub account, so you have every right to push to it)
• signal any interesting contribution back to the original GitHub repo (that is the "pull request" part)

Check also "Collaborative GitHub Workflow".

If yo want to keep a link with the original repo (also called upstream), you need to add a remote referring that original repo.
See "What is the difference between origin and upstream in github"

http://stackoverflow.com/questions/6286571/git-fork-is-git-clone