If you hit the history tab while viewing any article, you will be shown a complete history of the article: the last several revisions are listed chronologically, with the date the change was made.

If you click here on the Compare selected versions, you will be shown the differences between the current version of the article, and the previous. You can select between which versions you wish to see differences (you can for example pick the original version and the current version).

Doing so will give you something like this:

The red means the current version has something that was deleted compared to the previous one. The green means something was added. If you see only red, something was deleted. If you see only green, something was added. If you see both, something was changed.

If you click undo, the change you're looking at will be undone.