Classes in isolation would not provide much insight into how a system is designed. UML provides several ways of representing relationships between classes. Each of UML relationship represents a different type of connection between classes and has subtleties that aren't fully captured in the UML specification. When modeling in the real world, be sure that your intended viewers understand what you are conveying with your various relationships. We say this both as a warning to the modeler and as a slight disclaimer that the following explanations are our interpretation of the UML specification. For example, the debate over when to use aggregation versus composition is ongoing.? Again, the important thing is to be consistent within your model.
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