So this adds four new campaigns to the game, and some other stuff I don't entirely remember. The campaigns are fun, focused on specific areas/eras, and can provide something players of Medieval II's grand campaign could not provide with this level of detail.
Britannia lets you fight it out with England, Scotland, Ireland, and Wales in the late 1200's and early 1300's, which means that this scenario covers the struggles between England's Edward Longshanks and Scotland's William Wallace. The Crusades focuses on the most famous period of that conflict when personalities like Saladin and Richard the Lionheart ran around. Teutonic covers another crusade in the form of the Teutonic Knight's vicious assault upon Eastern Europe. Lastly, The Americas lets you play with the Aztecs just when the Spanish show up to createa new empire... with guns!
All of these campaigns are cool in their own way, with new factions and units and lots of opportunity for excessive violence. When you get bored of these however, you have the option of checking out some of the best and most thorough mods ever made for any game. There are plenty to choose from, but Third Age: Total War is probably the best of the bunch. So when the English, Arabs, or Teutons get boring for you, you can play with Gondor, Rohan, Elves, and Mordor's endless hordes instead. And it is awesome.
If you want, or have, Medieval II, you should get this too. The campaigns it comes with are plenty good, but the mod options add so much replayability that you could find yourself playing this game for a very long time. :o