Here is a nice simple explanation of Service Oriented Architecture (SOA):
Sounds a lot like distributed computing: Componentize your functionality and make it available on a network. I think the main difference with modern SOA is that you wrap your components with Web Services so the calling protocol is standard and the component’s platform doesn’t matter. Older “SOA” used CORBA and DCOM, which were proprietary and platform/language specific. Now we have web services, SOAP and REST which don’t care about the platform and WCF which does.
Am I being too simplistic?