In the simplest of terms, a form is a fillable document. It asks questions and includes space for recording answers.
Forms may be online (web-based) or offline (paper-based). A 'webform' refers to an online form. But the word isn't used very often. In most cases, we simply say 'form' or 'online form'.
A form builder refers to a drag-and-drop platform for creating online forms. It helps you quickly build forms, share them with your audience, and accept submissions.