What is difference between form_for and form_tag?

Hello!

The many answers of this question are not appropriated for me.
What is the BIG difference between form_for and form_tag?

In several web pages, i read like below...
form_for method is used for specificated wrapped model object,
and form_tag is just output the <form> tag.
It's not clear to me...

Isn't it equal that source using form_for or form_tag?

Please, any feedback to me.
Thanks!

Rucy.