form_for v/s field_for

I am trying to use complex-form in my application for which i have to
use field_for i want to know exactly the main difference of form_for and

they are both to do with binding an object to an instance of form
builder, but form_for also outputs the <form> tags. If you thing of
form_for being

"<form #{various form level options}" + fields_for(...) + </form> then
you're not far off.