In rails application usually we will be using <%=yield%> for dispalying the content in the block.
Our Rails Layout will something like this

<html>
<title>Content_for</title>
<head>
<%=stylesheet_link_tag 'application'%>
</head>
<body>
<div id='header'> </div>
<div id='content'><%=yield%></div>
<div id='sidebar'> </div>
<div id='footer'> </div>
</body>
</html>

If you want to change something in the layout on a per-template basis, content_for is your answer! This allows templates to specify view code that can be placed anywhere in a layout.
Now we want to include one more stylesheet for our index.html.erb,that should be used only for index.html.erb & not for other.

Assume there are two controllers

1.light

2.image

Both controllers are using the same layout,but each controllers need different css & stylesheets.For this purpose we will be using content_for

Example

In this example,content_for have been used & it is called in the layout itself,so the brower is showing the block content.

Code

Browser

Similarly we can use this for diff controllers.

1.Define our Layout

View code

2.Define content_for in light controller

View code

Now content_for has been defined for the light controller for index action.

Here the css & js files have used only for index.html.erb.so other actions wont be able to access the style & css