The RadWindow is actually an IFRAME element and it is used to load an external page in it as Princy explained. This means that you should put all the desired content in a separate [age and then show it in the RadWindow - the content will not be part of the same document but it will be another document. You can find the RadWindow's demos below - please examine them in order to get a better impression of the functionality this control offers:
Another popup control which can have content directly into it which will be part of the same document is the RadToolTip control. You can find its demos below:
Please, examine the suggested controls and choose the one which best fits your particular needs. In case you have further questions concerning the choice of a more suitable control, provide more details about your exact scenario and we will do our best to give you a proper advice.
All the best,
the Telerik team