Both types of questions are made for the collection of demographic data. For example a name or an email address. But both differ.
Use intro fields when you want to automatically insert data in your questionnaire. Ask your respondents to follow a link that already includes some extra data. E.g.: "su.vc/quiztitle/?cf1=TeamA". In this case “TeamA” will automatically be shown in the intro screen as one of your custom fields. In the results pages you will be able to filter/visualize data based on this intro field (e.g.: I only want to see the results for TeamB). For more information about intro fields please read Intro Fields andPre-fill Intro Fields.
An example of how the responses of the Intro fields looks like:
If you want the respondent to leave his contact data (for example by presenting a tablet during an event) then you better use Form Questions. Several formats are available: email, date, drop-down menu, numbers, text area, opt-in.