they basically change the carrier support or carrier version for your phone. for instance the carrier on your iphone 3G with 3.0 installed should be AT&T 4.0 (assuming you have AT&T). but to get tethering and MMS you need AT&T 5.0 so you can install this with a carrier update which come in a file called ATT_US.ipcc. you install this with itunes manually or when AT&T gets their lazy a$$ around to it they will release an update that itunes should automatically find and install for you