form_for :

we used form_for for a specific model i.e while we need to
create a new row in database. so, form_for perform the standard http post method and posses active record objects.

Below is the example for using form_for form helper:
view sourceprint?

<% form_for @client do |f| %>

First name: <%= f.text_field :firstname %>

Last name : <%= f.text_field :lastname %>

Email : <%= f.text_field :email %>

Address : <%= f.text_area :address %>

<% end %>

here we are using f object of model class “Client” to create form input fields

form_tag :

form_tag just creates an form as an normal form. form_for
will perform the standard http post without any model
backed and has normal fields. this is used mainly when
specific things need to be submitted via form

here is the example for using form_tag in ruby on rails:
view sourceprint?

<% form_tag ‘/posts’ do -%>

<%= text_field_tag “post”, “firstname” %>

<% end -%>