What is the difference between Web.config and Machine.Config in .NET?

Web.config file is used to make the settings to a web application, whereas Machine.config file is used to make settings to all ASP.NET applications on a server(the server machine).

What is the difference between a session object and an application object?

A session object can persist information between HTTP requests for a particular user, whereas an application object can be used globally for all the users.

Which control has a faster performance, Repeater or Datalist?

Repeater.

Which control has a faster performance, Datagrid or Datalist?

Datalist.

How do we add customized columns in a Gridview in ASP.NET?

Make use of the TemplateField column.

Is it possible to stop the clientside validation of an entire page?

Set Page.Validate = false.

Is it possible to disable client side script in validators?

Yes. Simply EnableClientScript = false.