What are the main differences between these WPF controls?
And when I should use a ToolTip instead of Popup?
A ToolTip is a small pop-up window that appears when a user pauses the mouse pointer over an element; the Popup control provides a way to display content in a separate window that floats over the current application window relative to a designated element or screen coordinate.
A Popup does not automatically open when the mouse pointer moves over its parent object. If you want a Popup to automatically open, use the ToolTip or ToolTipService class.
For more information see: ToolTip overview and Popup overview.
Previous tooltips in Winforms were overly restrictive in what you could display. This is why people rolled their own. WPF tooltips allow you to apply a templatize view with color, images, formatting, you name it. The days of rolling your own in that are mostly gone; mostly.
来源：https://stackoverflow.com/questions/11073470/tooltip-versus-popup-wpf-control