CONCEPTS
Take a clone of a remote repository and run git branch -a (to show all the branches git knows about). It will probably look something like this:
* master
remotes/origin/HEAD -> origin/master
remotes/origin/master
Here, master is a branch in the local repository. remotes/origin/master is a branch named master on the remote named origin. You can refer to this as either origin/master, as in:
git diff origin/master..master
You can also refer to it as remotes/origin/master:
git diff remotes/origin/master..master