Anil,
You are more or less on the right track. However, the terminology of Enterprise Services usually will not restrict itself to one single business function.
If I take the exampe you have quoted - PO Creation is a single business step, so its as good as a stand alone web service.
The differentiation comes from the stand point of accessing multiple disparate systems for different functionality and achieving one final business process. The quick example that I have seen coming time and again is that of CREATING a SALES ORDER. So, the BAPI / FM which creates the Sales order has been exposed as a web service. However, there might be other things that you want to before you actually create the order - like checking the Credit history of the customer. So, in this case there another stand alone web service which will give you the credit history of the customer from some external system (maintained by banks / Credit unions).
But if you look at it from stand point, its one single action that does both checking the credit history and depending on the reply from there goes ahead places a sales order(In a R/3 system). However, it might not stop there becase you might have the required stock. So, it might as well go raise a PO (In some legacy procurement system) to your vendor or create a production order (Some other inhouse developed production order system).
ESA - is concept where in the orchestration of individual web services is done to achieve final business goal.
Hope this clairifies questions around ESA.
Regards,
Ravi
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