I develop a desktop application, and I'd like to add a "question mark" icon besides one of the textbox (that describes the meaning of that textbox).
My question is what's the common way to use the question mark: Should I show the text when hovering on the question mark? e.g. using a tooltip? Should I show the text just after clicking on the question mark?
Thank you!
I suggest you show the tooltip on hover. That is one click less for me as a user.
That aside another thing to consider while constructing a forms based user interface is to label the controls descriptively enough such that users don't need any further help beyond the component label. If the tooltip is your primary means of conveying the meaning of the control, you definitely have to rethink the form design. Basically don't use tooltips as an affordance, a placeholder with 2-3 words as additional description is a better first choice.
The only place where I see click to reveal tool-tips being really useful is in a touch interface.