I think you are referring to a rich text field. This type of field allows the user to enter information as well as indicate font, style, color etc ... for their input. This is only possible if the designer of the form allows it, otherwise they are only able to enter information in the font, style etc that the designer chose. The user woudl hit Ctrl-E to bring up the rich text menu and then they ghave the tools to affect the text.
Paul