In the Linux world, a service (also known as a daemon) refers to a single program that runs in the background and typically listens on a port to respond to service requests. An OpenStack service, on the other hand, refers to a collection of Linux services working in concert.
OpenStack services are implemented by multiple Linux services. For example, nova-compute and nova-scheduler are two of the Linux services that implement the Compute service. OpenStack also depends on several third-party services, such as a database (typically MySQL) and a message broker (typically RabbitMQ or Qpid).
In this document, we generally use the term "service" to refer both to lower-level Linux services and higher-level OpenStack services. It should be clear from the context whether we are referring to a high-level OpenStack service (e.g., Image), or a low-level Linux service (e.g., glance-api).
