What relationship is between the whale and barnacle?

The relationship between a whale and a barnacle is an example of commensalism. Commensalism is a type of symbiotic relationship in which one organism (the commensal) benefits from the association with another organism (the host), while the host is neither harmed nor benefited.

In the case of the whale and the barnacle, the barnacle benefits by attaching itself to the whale's skin, where it can access a steady supply of food and protection from predators. The whale is not harmed by the barnacle's presence, and may even benefit from it by having its skin cleaned of parasites.