I would like to know how does the login procedure work in the Internet sites (e.g. Gmail). I am trying to implement an online application which I want to secure with user authentication.
Could someone give a detailed explanation or point to a relevant site
Many websites use OpenId to allow users to login. OpenId uses OpenId Providers (such as Google) to login. One big advantage is, you do not have a password for the user stored (so if you get hacked, it can’t be leaked). Some info: http://de.wikipedia.org/wiki/Openid
ASP.Net has the concept of Membershipproviders, which can in turn use many different ways. See here: http://msdn.microsoft.com/de-de/library/system.web.security.membershipprovider.aspx
There are many ways (another may be OAuth) to create a login, I think you can’t talk about one single correct way.