You've got to focus on the distinct XML/WS libraries: JAXB, SAX, StAX, JAXP, JAXR, etc.
You must also know how a WS can be integrated in a transaction and how it can be secured.
From an larger "integration" point of view, this article from IBM
was useful for me: it gives you the keys for choosing among WS, JMS and JCA.
Hope it helps,