form_tag and form_for provide two very similiar interfaces to much of the same thing. Keeping with the DRY principle, repetition is shunned upon and soon form_tag and form_for will be depricated and replaced with with form_with.
form_tag is used to create forms where there is no underlying model.
<%= form_tag users_path do %>
<%= text_field_tag :email %>
<%= submit_tag %>
<% end %>
form_for is used when you have model.
<%= form_for @user do |form| %>
<%= form.text_field :email %>
<%= form.submit %>
<% end %>
here is an example for form_with without a model.
<%= form_with url: users_path do |form| %>
<%= form.text_field :email %>
<%= form.submit %>
<% end %>
Here is an example with a model
<%= form_with model: @user do |form| %>
<%= form.text_field :email %>
<%= form.submit %>
<% end %>
Setting the model argument then scope and url are automatically derived from it. this pattern is derived from form_for.
The diffference in syntax between the two examples provided above is that form_for uses form builder helpers making form_tag a little more efficient in regards to the amount of code required to complete a form. form_with will provide form builders.
The unification of usage and subsequent calls to field tags will simplify how you build your forms. Additionally, from_with solves some deficiencies with the current helpers.