If there is no diamond, then we have a simple association.
If the diamond is left empty, it signifies it is an aggregation. This relation is stronger than a simple association. In this case a Customer aggregates Orders.
If the diamond is black, this means it is a composition, which is even stronger than an aggregation because the composite class cannot be composite by other classes. Its “life” depends on the container.
I think it’s explained a little bit more clearly, at:
- Ezra, Aviad (May 28, 2009) “UML Class Diagram: Association, Aggregation and Composition“