The SilverLight Popup control can hold a single piece of content, which can
be any Silverlight element. (This content is stored in the Popup.Child
Property, rather than the ToolTip.Content property.) Popup can be shown or hidden using the same IsOpen property.
The differences between the Popup and ToolTipare
1) The Popup is never shown automatically. You must set the IsOpen property for it to appear. The Popup does not disappear until you explicitly set its IsOpen property to false.
2) The Popup can receive focus.so you can place user-interactive controls
in it, such as a Button , CheckBox etc.,. This functionality is one of the key reasons to use the Popup instead of the ToolTip.
3) The Popup is placed at the top-left of the page. You can move it down and
to the left using the HorizontalOffset and VerticalOffset properties.