You've already seen what Entities and Value Objects are. As basic building blocks, they should contain most of the business logic of any application. However, there are some scenarios where Entities and Value Objects aren't the best solutions. Let's take a look at what Eric Evans has to say about this in his book, Domain-Driven Design: Tackling Complexity in the Heart of Software:
When a significant process or transformation in the domain is not a natural responsibility of an Entity or Value Object, add an operation to the model as standalone interface declared as a Service. Define the interface in terms of the language of the model and make sure the operation name is part of the Ubiquitous Language. Make the Service stateless.
So when there are operations that need to be represented, but Entities and Value Objects aren't the best place, you should consider modeling these operations...
