What your looking at may be a subset of a revision control server.
If you heard of it, CVS, or Subversion are 2 of the most known of them.
A RCS program allows you to "commit" modifications of your work, and merge it with other modifications done by other peoples.
Definition and capabilities of RCS programs:
Think about multiples web dev. working on the same PHP application here.
One working on a part of the project can update his sources to incorporate what others have done, and he can commit his work, which will allows others peoples to get it too next time they will make an update.
What makes RCS programs a real killer, even when you're working alone, is that they allows you to get a snapshot of your application at any given time.
For example, you can ask the server to update your source to the state they where 3 days ago, spot a change that was done in the mean time, export a patch to be applied against the current source, revert to the actual state and re-incorporate the patch in your file.
This saved me numerous times, when I had updated the productions server with a file not ready for production.
I simply asked the server for the previous "ok" version to get it running back ok.
I even keep some important files under that. Like my SSH keys, my linux configurations scripts, my firefox bookmarks and so on.
Like that, I can always get an up to date version of those files anywhere I've got a net connectivity.
Anyway, back to your question:
If you want to compare the differences between 2 files, you could simply use a "diffing" program.
It will load the 2 versions of your file, and show you where are the differences between them.
On windows, I've used winmerge in the past:
In your case, it would surely be easier than setting up a CVS server.
But if you want to go that way, ask me, as I've implemented both CVS and subversion on my server, and I'm using them daily.
Links:
http://winmerge.org/
http://drupal.org/node/324
