Web Services enable the exchange of data and the remote
invocation of application logic using XML messaging to move
data through firewalls and between heterogeneous systems.

The key to understanding Web Services is knowledge of the
underlying protocols. Importantly, by default, all
communications between Web Services servers and clients is
through XML messages transmitted over the HTTP protocol.

SOAP (Simple Object Access Protocol) is the protocol that
allows us to encapsulate object calls as XML.

Disco and UDDI
You need to know where and how to locate a Web Service in
order to be able to use it - a process known as discovery.
The aim of these two protocols is to facilitate the
discovery process. 

WSDL
WSDL (WebServices Description Language) does what it says -
allows description of the Web Service - it specifies the
SOAP messages that it can send and receive.

System.Web.Services.WebService.