What is Total War: Rome 2 Emperor Edition?

I have just noticed that in my Steam library, the game Total War: Rome 2 is now called Total War: Rome 2 Emperor Edition. What is the reason behind the edit to the game name? Have they added anything to the game or is it purely just a name change?

There is a major difference between Total War: Rome 2 and Total War: Rome 2 Emperor Edition.

There is only one version of Total War: Rome 2 on steam, and it's the emperor edition. There is no regular version on steam any more, and according to the devs the emperor edition is a majorly revamped version, with loads of new features, and an expansion as well.

The emperor edition was given for free to all existing owners of the regular edition.

From Steam:

Big news! We’ve announced the Total War: ROME II Emperor Edition – the definitive edition of ROME II, FREE to all existing owners of the game, which comes complete with a large piece of new single player content. In Rally Point 21, we go into exclusive amounts of detail about what’s included.

Based off this Steam announcement by the developers, it appears that Total War: Rome 2 Emperor Edition is an expansion to the original game. The reason you have it:

If you’re an existing owner of ROME II and have automatic updates enabled, Steam will update the game to the Emperor Edition for free.

Among the many changes, the most prominent that I can see are:

• An additional campaign - The Imperator Augustus Campaign
• Armenia is now playable in the Grand Campaign

There are plenty of balance changes, fixes, and improvements as well. You can find all of the changes by looking at the patch notes, though there are too many changes to simply copy all of the notes into my answer.