Carcassonne is a medium-light game for 2-5 players that plays in about 30-45 minutes. Carcassonne: the Expansion or, as it became known when it became apparent that one expansion was not enough, Carcassonne: Inns & Cathedrals extends upon the Carcassonne experience without changing the game too much.

This first expansion adds a number of things to the game:

A Sixth Player

Carcassonne really works well with fewer players, 2-3 generally being considered optimum. Once you reach six players, the luck of the draw becomes far more important, although two players can co-operate to make the most of it. Of all the additions to Carcassonne, this can be considered the least important. However, for larger families just wanting a casual game, it's not a bad addition.

A Large Meeple

This addition gives each player eight meeples upon which to contest the field of play; however, it does more than that because the Large Meeple is worth two meeples when determining who controls features. This improves the play of terrain dominance, and also allows play to isolate the Large Meeple in an uncompletable feature.

Inns

Roads have a problem in regular Carcassonne: they aren't worth much. As a rule, you almost always want a city piece over a road piece, although roads can be used for blocking. Inns help solve that problem by doubling the value of a road (2 points/section). They're also somewhat risky, as an uncompleted road with an Inn on it will be worth nothing! As a piece to use on your opponent's incomplete road, they're rarely useful, but they work quite well on your own.

Cathedrals

Cathedrals, like Inns, make cities worth an extra point per tile & pennant (3 points rather than 2 points). However, they are far better used offensively to make an opponent's city harder to complete than to improve the scoring of your own city, due to the difficulty of completely large cities. The two features may look similar, but they play differently.

More Tiles

Apart from the Inns and Cathedrals tiles, there are a few other tiles in the expansion. These fill in a few "gaps" in the original tile set, allowing more combinations of sides. As a result, "blocking" moves that dictate three or four sides of a tiles placement become less reliable; good Meeple placement becomes more important.

Another side effect of the new tiles is to break up farms a little more. This is particularly desirable due to an occasional problem with all the meeples ending up in one big farm connected to all the cities!

Scoring Tiles

These six tiles exist simply to help you keep track of when you've reached 50 or 100 points (or even 150 points in really big games). They're printed with 50 on one side and 100 on the other, and you take them when you "lap" the scoring board.

Conclusion
When you get down to it, Inns & Cathedrals is a subtle expansion. It changes the game, but not so as to distort the Carcassonne experience, fixing a few problems in the base game along the way.

I don't really find it an exciting expansion, and I find that the second expansion, Traders & Builders, gives a much better play experience, but it's an enjoyable addition to the game.