ContentPlaceHolder in master page head

 ContentPlaceHolder control is used to define a region for content in an Master Page, to hold the  
 contents (asp:Content) of an associated Content control.

 On a master page we can have one or more ContentPlaceHolder controls.

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hi majid,

see master page is genarally used to make ur app to make maintainable easly

if u have set of pages in which u feel the outlook will be same just controls in some parts changes then to make the maintaince easy u go for master page concept in master page u create an out look which will be static for all the pages and content place holder is the one which tell u that page in heriting the master page control will use that area for changable datacontrol which are neeed for that perticular page 
for ex:

if u have the master page say masterpage.master 
here in masterpage.master u will have u entire static design lay out and empty content place holder

if i have in herited the masterpage.master to default.aspx then here in default .aspx i will get on content place holder where i just need to write the set of controls specific for that page but out look willl be copied from master page which in runtime will make u to get complete page


hope it will help u