title: How to use cancancan?
body: <p>I want to give rights to users in my rails app.  I have 'admin' who can create, update and delete all posts and comments, 'user' who can create and update only his own comments, and 'guest' who can do none of these.  For this I use the gems 'devise' and 'cancancan'.  I understand the 'devise' gem, but I don't understand 'cancancan'.  </p>
  
  <p>In the class ability.rb, how can I write rights for these users (admin, user, guest)?</p>