Frozen yogurt tends to have less fat and fewer calories than ice cream. Unlike regular yogurt, frozen is not subject to an official government "standard of identity." Some frozen yogurts contain live and active bacteria (look for an "LAC" seal), which may provide health benefits.
As you go down the ratings, the ice creams tend to be a bit more gummy or have flavor that's generic or less pronounced. The best frozen yogurts have a slightly eggy taste, a bit like vanilla custard. The also-rans tend to have more flaws, including a foamy or airy texture or slight bitterness. The Greek frozen yogurts are less sweet than most of the others, with a big sour-dairy flavor.