DevStudent

Download Ajax Control Tool Kit from below link

Ajax control toolkit download link

Extract The file then Go To Visual Studio 
Open Tool Box
Right Click On The Existing Tab
And Click On Add New Tab 
Name the tab
Right click on the newly tab
click on choose all items
Template Will Be Open And Select Browse Go To The Location
And Select AjaxControlTOolkit.dll  
Click Open It Will Open Security PopUp Window - Select Yes
Ajax controls will be added in toolbox which can be used to extend the existing functionality of ASP.NET controls.

Master pages are used to create common layout for multiple websites.
Master pages will have .master as page extension and we can see output of master page directly.
Multiple web forms(.aspx pages) can be created using single master page. All these web forms will inherit the UI and functionality from its master page.
Output of the master page can be tested by testing its child pages which inherits its master page UI and functionality.

When multiple pages are created using a master page, Common UI and behavior of all the web forms can be controlled from master pages.