Composition : Defines a strong-coupled relationship between
two entities, where the one entity is part of another, and
both need each other for their existence. e.g. Human body
and the Heart.

Aggregation : Defines a weak-coupled relationship between
two entities, where one entity could be part of another, but
either can exist without the other, independantly. e.g.
School and teacher.

Composition can be used to model by-value aggregation which
is semantically equivalent to an attribute. In fact
composition was originally called aggregation-by-value in an
earlier UML draft with “normal” aggregation being thought of
as aggregation-by-reference. The definitions have changed
slightly but the general ideas still apply. The distinction
between aggregation and composition is more of a design
concept and is not usually relevant during analysis.