When starting to learn Domain Driven Design, entities and values objects are things people normally understand pretty quickly. But, when it comes to services, I feel things are a bit more unclear for many. If you ask a Domain Driven Design expert about services, you probably will get the answer; a service is something you can’t find a good place for in your domain model.
A concrete example
In almost every modern GUIs you use combo boxes as a way to display different choices for the user. You need a way to fill these boxes (look at the figure), but the action where you fill them doesn’t fit into your domain model in a natural way. I look at this as a service, something you need in your domain, but nothing you will force into your domain objects. In the applications I have created lately, I have often used value objects to transfer the data from the repository to the GUI through a service (follow the red arrows in the figure). You can of course create dedicated objects for this (follow the orange arrows in the figure), but I haven’t found the need for this yet.