In grammar the superlative of an adjective or adverb indicates that an entity transcends at least two other entities in some way. For example, if Jim is 25, Jack is 35, and John is 40, John is the oldest of the three, because his age transcends those of Jim and Jack in one direction, while Jim is the youngest, because his age transcends those of Jack and John in the other direction. If Joe, who is 45, and Jules, who is 20, join the group, Joe now becomes the oldest and Jules the youngest. (Strictly speaking, if a group contains only two entities use of the superlative is ungrammatical: if the group contained only Jim and Jack, Jim would be "the younger", while Jack would be "the older", but it's common for "the youngest" and "the oldest" to be used instead.)
In English, the superlative and the comparative are created by inflecting adjectives or adverbs. The structure of a superlative consists normally of the positive stem of the adjective or adverb, plus the suffix -est, or (especially in longer words) the modifier "most" or "least" before the adjective or adverb. It always has the definite article and is completed by "of" or other preposition plus one or more nouns of entities that it surpasses to the highest or greatest degree, such as in "he is the tallest of/in the class", or "the town is the most beautiful of all in the country".
Mention should be made also of the elative, if not an actual separate inflection but the intensified degree of adverbs and adjective; e.g. "I am the greatest", or "she is most beautiful". Intensification in these cases means "really great" or "very beautiful indeed", and is usually used without relating it to other persons, things or entities which a normal superlative does.