Both RadTooltip and RadTooltipManager can display rich content (including user controls and other ASP.NET controls), as well as AJAX-generated content. RadToolTip is meant to "tooltipify" a single element while RadToolTipManger should be used in scenarios where many elements would require a tooltip. For more information on RadTooltipManager, see the RadToolTipManager Overview topic.
The two most common RadToolTip properties are:
TargetControlID: the ID of the control for which the tooltip should appear on the client
IsClientID: True if the TargetControlID is a client id.