InfoPath Dev

I have a question in a form that a user will fill out. Beside this question,  I would like to put a button with a question mark (like help in windows). When users click on this question mark-button, it will open up a temporary popup which will tell them more about how to answer the question and also this popup can have links to external webpages.

Whats the best way to go about this?

A pop up will require that you write code.

A simpler option is to have a hidden section with help text. The section is hidden by default when the form opens. When the user clicks the button the section is shown.

Good luck!

 Is it possible to rehide the help text , if the user wishes to, after reading ?

Sure, just have two option buttons to toggle: Show Help and Hide Help.