From within the designer:

1) From the ToolBox, drap a ToolTip control to your form.

2) Check the properties of this toolTip1 object to make sure Active is set to true.

3) Click on your control (e.g., a button), and in its Property page, set the ToolTip on toolTip1 entry.

From code:

using System.Windows.Forms;

private ToolTip toolTip1; 

toolTip1 = new ToolTip( components ); 
toolTip1.Active = true; 
// ...

toolTip1.SetToolTip( button1, "Press for information..." ); 

Contributed from George Shepherd's Windows Forms FAQ