Ruby on Rails

Hello, 

I am very new to Ruby on Rails.  I am working my way through the guides at rubyonrails.org.  It isn't going very quickly but I am determined to make it work. 

Here is a very basic question but I just can't figure it out by looking at the examples.  It is regarding embedded ruby.  

What is the difference between these two forms of embedded code. 

<%=  ... some code ...  %> 

and 

<%  ... some code ... %> 

Some lines have the equal sign in it and most of them don't.  I just want to know what the rule is for using either. 

Thanks. 

Dan