Form workflow usually follows these stages:
This workflow actually includes some of the nodeapi hooks as well as FAPI workflow to help illustrate the places where form elements can be
Widgets are the parts that make up forms, like textarea widgets, file upload widgets etc.
Drupal 7 forms that modules create can be changed by other modules and even themes with the help of Drupal FAPI hooks system. See Drupal 7
Explain the use of #tree and #parents properties in a form array.