The Positive Degree of an adjective in comparison is the adjective in its simple form. It is used to denote the mere existence of some quality of what we speak about. It is used when no comparison is made. Ex: It is a tall building.
The Comparative Degree denotes the existence of a higher degree of the quality than the positive. It is used when two things (or two sets of things) are compared. Ex: This building is taller than any other building.
The Superlative Degree denotes the existence of the highest degree of the quality. It is used when more than two things are compared. Ex: This is the tallest building.