Association is any relation between classes where instances of one class have a field reference to an instance of another class.
Composition is a "stronger" relation, meaning that one instance (parent) "owns" another one (child).
It is Aggregation that doesn't have any additional semantics other than being an association.
See more here: http://martinfowler.com/bliki/AggregationAndComposition.html
EDIT: You may add some special semantics to aggregation symbol, like "May be owned by maximum one parent at a time, but may change parents, or be an orphan." However, such extensions are your own, and are not defined in UML, as far as I know.
