Aggregation is a strong form of association. It represents a part-of relationship.
It is transitive, i.e., if A is a part of B, and B is a part of C, then A is also a part of C.
It is asymmetric, i.e., if A is a part of B, then B is not a part of A.
For example, a microcomputer is modeled as the aggregate of a sound card, an I/O controller, a CPU, etc. Each component is independent of the whole, i.e., a microcomputer, but its existence is more relevant for its use in the microcomputer as a composite whole, rather than its separate existence as a part.
In UML, an aggregation relationship is shown symbolically as a line and a diamond at the end of the association link.
The image below depicts the aggregation symbol:
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