A service is composed of a set of features (attributes, commands and events) that can be used by objects of different types and by other services. All features in one service belong to a logical group of features. For example, a math service contains features used in mathematical computations; an accounting service contains features used by an accounting application.

A service is like a type of which there exists exactly one object.

A service doesn't have to be created such as an object that is created by a factory. Services are always available, without the need for creation or activation.

examples:

A math service provides commonly used mathematical constants like pi = 3.1415926..., e = 2.7182818...

An error_logger service logs errors and can output the error list in different formats

For more information about services: see the section called “Service”