The orchestration of services is the task aiming at invoking a set of services in a structured way. It consists in a centralized entity, called the orchestration engine, that can be seen as a conductor driving his musicians.
The orchestration engine uses control constructs in order to build an execution graph where main nodes are services invocation. There are, for example, conditional constructs, loop constructs, flow or sequence constructs, as you can see in the following figure.
Example of execution graph extract.
In the context of SOA, the OASIS consortium proposed the WS-BPEL standard as a language for services orchestration (the last version is 2.0).
This standard uses Web Services well known mechanisms and in particular WSDLs (1.1) for services definition.
Indeed, an execution graph, or process, or workflow, written in WS-BPEL is itself a (Web) Service, defined thanks to one ore more WSDLs.
Let's go back to the example of execution graph. In WS-BPEL, we can add 2 main nodes, a Receive (and Pick) activity and a Reply.