I have looked on the web but I am confused as to whether a wrapper service wraps a legacy application, or a set of complex services.
Update: Judging by the answers I have received there is no accepted definition for a "wrapper service" in SOA, and after further investigation I realised that "wrapper services" depends on who defines it. I mean in an Oracle context.
I understand a wrapper service as a service that wraps something, be it another service, a set of services, or a legacy application.
"wrapper service" which is also more widely known as "legacy wrapper" is used to wrap a legacy component to prevent coupling issues in soa.
hiding complex systems or a set of services is done using the "facade service"
Arguably the entire point about SOA is that everything is a wrapper. That is , a web service is just a facade, and there is no way to tell whether it fronts a "legacy application" or "a set of complex services".