form_for and form_tag, when to use which one?

Question

I am reading a Rails book and so far it has been using form_for to create the forms. In the new chapter that it is building a login page for users, it is using

Answer 1

In most cases when you are sending data to your database with a model, in methods like create, update, destroy. For this type of action you can use form_for.

Instead form_tag only create a simply form html, for example, a search:

 <%= form_tag("/search", :method => "get") do %>
   <%= label_tag(:q, "Search for:") %>
   <%= text_field_tag(:q) %>
   <%= submit_tag("Search") %>
<% end %>