String cheese is mozzarella cheese that is formed into cylinders. It has the property of being able to be taken apart into smaller and smaller fibers. The above writeup is incorrect in saying that it is processed cheese: string cheese is mozzarella cheese, and often good quality mozzarella cheese at that. Most mozzarella cheese exhibits the same property, it is just that the shape of string cheese accentuates this property.
Most pieces of string cheese weigh about an ounce, and at 80 calories and 6 grams of protein per piece, is a good compliment to the type of carb and fat heavy foods that make up most snack foods. String cheese also usually costs around 50 cents a piece, although sometimes as little as 25 cents Scaled up to the pound, this is 8 dollars per pound, which is too much, but you pay premium for the fun of getting to dissect your cheese before you eat it. If you are paying more than 80 cents to a dollar for a piece of string cheese, chances are that you are either getting it from a bourgeois deli or a really unbourgeois gas station or minimarket, both of which are of course not cool. Carrying a piece or two of string cheese with you is a vital part of most day trips and adventures.