If you see a post that has been edited, if you click on the "Edited [Date]" link you will see a list of post revisions. I'll use your question here as an example.
If you click on the link as shown in the red box:
You can see (in red) what was taken out and (in green) what was added.
For more info, have a look at this FaQ entry, "How Does Editing Work?"
