degrees of comparison

Read the following sentences:

1. John is tall.
2. Peter is taller than John.
3. Harry is the tallest of the three.

In sentence 1, the adjective tall merely says something about John’s height. It doesn’t state how tall John is. In sentence 2, the adjective taller is used to compare John’s height with Peter’s height.
In sentence 3, the adjective tallest is used to compare Harry’s height with the height of John and Peter.
We have thus seen that adjectives change in form to show comparison. These different forms of the adjective are called the degrees of comparison.
In the examples given above, the adjective tall is said to be in the positive degree. The adjective taller is said to be in the comparative degree and the adjective tallest is said to be in the superlative degree.
The positive degree of an adjective is the adjective in its simple form. It is used to denote the mere existence of some quality. Adjectives in the positive degree are used when no comparison is made.
The comparative degree of an adjective shows a higher degree of the quality than that is present in the positive degree. It is used when two things or two sets of things are compared.

• Peter is smarter than John.
• Which of the two sisters is the prettier?
• Apples are dearer than oranges.

The superlative degree of an adjective denotes the highest degree of the quality. It is used when more than two things or sets of things are compared.

• Peter is the smartest boy in the class.
• Iron is the most useful of all metals.
• Alice is the prettiest girl in the neighborhood.

source : http://www.englishgrammar.org/degrees-comparison/