Difference between ScriptManager and ScriptManagerProxy in asp.net
You can only have one script manager per page but you can have multiple proxies.
Used if you have the script manager in your master for instance which will go across all pages but you want a script manager reference in an underlying page or user control. In that instance you would add the proxy. If you added another script manager the page would error.
The offical MSDN documentation gives the details.
The diffrence between "ScriptManager" and "ScriptManagerProxy" is only one , ScriptManagerProxy enables nested components to add script and service references if the page already contains a ScriptManager control , whereas ScriptManager includes the Microsoft AJAX Library and the functionality that enables partial-page rendering.Visit this, go to botton of the page :)