There are 3 main classes of custom fields: Selection, Text entry and Options:
Selection fields provide fixed lists of items or terms that users select from either a dropdown menu or checkbox listing.
Text entry fields allow users to add their own information, and include a short text box (100 characters), a rich-text field (unlimited, with simple formatting options and hyperlinking), and a URL field (automatically hyperlinks any web address entered).
Options fields include radio buttons (e.g., yes/no options) and a checkbox (e.g, opt-ins).
There is no limit on custom fields. Our HowTo Guide details Custom fields and how to set them up.