I am using Visual Studio 2012 with TFS. Whenever I want to compare my checked out files, I have two options to choose from:
To restate the Answer:
The local file is always the one you are working on.
The server file can either be:
The latest version including all other check-ins. (You see your changes, as well as other peoples changes.)
The Version as it was when you checked it out (You see only your changes, even if other people have checked in since).
If no other people check in files since your checkout, then the both of the above produce exactly the same result.
So if you want to compare what you have done against what it was when you started, select Workspace Version
If you want to compare what you have done against what other people have done, select Latest Version