The ContentPlaceHolder is an element that should be placed in the master page. This defines a region that content pages can fill in. The Contentcontrol should be placed in a content page that is based on the master page. It is used for supplying the content for the ContentPlaceHolder element that it is connected to the master page.
<asp:Content ID="Content1" ContentPlaceHolderID="IdOfContentPlaceHolder" Runat="Server"></asp:Content>
When the page is returned to the server to evaluate code handlers, and the same page is sent back to the browser afterward.
When an event happens on your page that causes the page to return to the server, handle the events, and then send the same page back to the browser. The contents of the page may have changed, but the page object itself is the same.
The two types of postbacks are synchronous and asynchronous. Synchronous in which the entire page is sent back to the server as with a non-ajax page. Asynchronous means, in which only part of the page is updated and the rest is unaffected as with ajax page.