Ali Arsanjani is Chief Architect from IBM (SOA and WebServices center). This article posted in Dec'2004 is still up to date and it is a good start point if you are new in SOA. Starting with the conceptual model of a SOA architectural style, showed in the figure below:
And this is the Ali's vision about the layers in a SOA typical architecture (and believe me it is valid today!):
A good article about the basic SOA principles and components definition.