Web Services with ASP.NET

Web services provide infrastructure to create distributed computing applications. Simply web services resides in a web server and enables clients to invoke its methods. In .NET, clients can communicate with web services using proxies. Usually HTTP handlers are used to implement web services.

Source : developer.mulesoft.com

You can generate the proxy class using WSDL.exe if you have the web service description which conforms to WSDL. When you use this tool a single proxy class is generated. 

See creating an XML web service proxy (MSDN)
How to generate web service out of WSDL (SO)

WSDL (Web Service Definition Language)

Describes the web service

SOAP

It is a programmable application logic accessible via standard Web protocols. SOAP (Simple Object Access Protocol) is one example. SOAP uses XML for data description and HTTP for transport. 

SOAP message consists of,

• SOAP envelop
• SOAP headers
• SOAP body

RESTful Web Services

RESTful web services does not contain any contract or WSDL file. It is simple to implement and supports different data formats (JSON, XML etc.)

Serialization in Web Services
Web Services use XMLSerialization for serializing response. 
https://msdn.microsoft.com/en-us/library/564k8ys4(v=vs.110).aspx

Resources

http://msdn.microsoft.com/en-us/library/ms972326.aspx