Capture lets you extract parts of code which can be used in other points of the template or even layout file.

Capturing a block into an instance variable

<% @script = capture do %>
  [some html...]
<% end %>

Add javascript to header using #content_for

#content_for("name") is a wrapper for capture which will make the fragment available by name to a yielding layout or template.

layout.rhtml:

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
    <title>layout with js</title>
    <script type="text/javascript">
      <%= yield :script %>
  </script>
</head>
<body>
  <%= yield %>
</body>
</html>

view.rhtml

This page shows an alert box!

<% content_for("script") do %>
  alert('hello world')
<% end %>

Normal view text
Methods
C
Instance Public methods
capture(*args, &block)

Capture allows you to extract a part of the template into an instance variable. You can use this instance variable anywhere in your templates and even in your layout.

Example of capture being used in a .rhtml page:

<% @greeting = capture do %>
  Welcome To my shiny new web page!
<% end %>

Example of capture being used in a .rxml page:

@greeting = capture do
  'Welcome To my shiny new web page!'
end

Source: show 

# File rails/actionpack/lib/action_view/helpers/capture_helper.rb, line 56
def capture(*args, &block)
  # execute the block
  begin
    buffer = eval("_erbout", block.binding)
  rescue
    buffer = nil
  end
  
  if buffer.nil?
    capture_block(*args, &block)
  else
    capture_erb_with_buffer(buffer, *args, &block)
  end
end
content_for(name, content = nil, &block)

Calling #content_for stores the block of markup for later use. Subsequently, you can make calls to it by name with yield in another template or in the layout.

Example:

<% content_for("header") do %>
  alert('hello world')
<% end %>

You can use yield :header anywhere in your templates.

<%= yield :header %>

NOTE: Beware that #content_for is ignored in caches. So you shouldn’t use it for elements that are going to be fragment cached.

The deprecated way of accessing a #content_for block was to use a instance variable named @@content_for_#{name_of_the_content_block}@. So <%= content_for('footer') %> would be avaiable as <%= @content_for_footer %>. The preferred notation now is <%= yield :footer %>.

Source: show 

# File rails/actionpack/lib/action_view/helpers/capture_helper.rb, line 92
def content_for(name, content = nil, &block)
  eval "@content_for_#{name} = (@content_for_#{name} || '') + capture(&block)"
end