Tooltip is a small pop-up window that is displayed when a user move the mouse pointer over an element.
The content of a tooltip can contain text, images, shapes, etc.
Example:
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525" Background="#FFF3EEEE">
<TextBox HorizontalAlignment="Left" Height="29"
VerticalAlignment="Top">TextBox with ToolTip
<TextBox.ToolTip><ToolTip>
<DockPanel >
<TextBlock Text="ToolTip Information"/>
</DockPanel>
</ToolTip>
</TextBox.ToolTip>
</TextBox>
</Window>