Dependency between classes means that one class uses, or has knowledge of, another class. It is typically a transient relationship, meaning a dependent class briefly interacts with the target class but typically doesn't retain a relationship with it for any real length of time.

A dependency implies only that objects of a class can work together.
The dependency relationship is often used when you have a class that is providing a set of general-purpose utility functions, such as in Java's regular expression (java.util.regex ) and mathematics (java.math ) packages. Classes depend on the java.util.regex and java.math classes to use the utilities that those classes offer.

Associations are stronger than dependencies and typically indicate that one class retains a relationship to another class over an extended period of time. The lifelines of two objects linked by associations are probably not tied together (meaning one can be destroyed without necessarily destroying the other).
Association means that a class will actually contain a reference to an object, or objects, of the other class in the form of an attribute.

Aggregation is a stronger version of association. Unlike association, aggregation typically implies ownership and may imply a relationship between lifelines.

Composition is used to capture a whole-part relationship. The "part" piece of the relationship can be involved in only one composition relationship at any given time. The lifetime of instances involved in composition relationships is almost always linked; if the larger, owning instance is destroyed, it almost always destroys the part piece.