A quick and simple overview about some of the basic web services technologies.
SOAP - Simple Object Access Protocol, is a protocol used for exchange data between client/server through web services using XML data format. An example can be seen here.
WSDL - Web Service Description Language, is a XML-based interface definition language used to describe the functionality offered by a web service. An example can be seen here.
REST - Representational State Transfer, simpler alternative to SOAP that uses HTTP methods like GET, POST, PUT, UPDATE and DELETE. An example can be seen here.