What is mean by WebService And what is the use of WebService in ASP.Net

WebServices : As the name implies, A WebService is an entity or a piece of code which is hosted on the Web, which serves a specefic purpose. This could be the most simple framed definition.

Refer this for How to add the Reference of WebService in WebSite.

How to add reference of Web Service (ASMX) in ASP.Net using Visual Studio

For basic refer these link:

http://www.codeproject.com/Articles/524983/Web-Services-From-a-beginners-perspective

http://www.codeproject.com/Articles/337535/Understanding-the-Basics-of-Web-Service-in-ASP-NET

Thank You.