Question

Does it have something to do with output?

So, <%= ...code... %> is used for outputting after code is executed, and <% ...code... %> is only used for executing the code?

Answer 1

This is ERB templating markup (one of many templating languages supported by Rails). This markup:

<% ... %>

is used to evaluate a Ruby expression. Nothing is done with the result of that expression, however. By contrast, the markup:

<%= ... %>

does the same thing (runs whatever Ruby code is inside there) but it calls to_s on the result and replaces the markup with the resulting string.

In short:

<% just run code %>
<%= run code and output the result %>

For example:

<% unless @items.empty? %>
  <ul>
    <% @items.each do |item| %>
      <li><%= item.name %></li>
    <% end %>
  </ul> 
<% end %>

In contrast, here’s some Haml markup equivalent to the above:

- unless @items.empty?
  %ul
    - @items.each do |item|
      %li= item.name