What is the difference between ice cream, gelato, and sorbet?
First of all, the names vary a bit from country to country, or the ingredients do.
I'll explain the most common names/ingredients.

The main difference is in the ingredients used. Sorbet is basically water + sugar + fruit, while ice cream and gelato is milk/cream + sugar + fruit. So the last two are more 'creamy', while sorbet is more 'icy'.

You can say that ice cream is a type of gelato, but there still is a difference. More sugar in gelato, more butterfat (the percentage of fat in the milk/cream) in ice cream.

You can read quite a lot about ice cream on wikipedia.