Default application action

I'm kinda new to RoR and I wondered how do I define which controller and
action is run by default when you point the browser to the application
address (ie not specifying a controller name)?

Look at config/routes.rb

Near the top you'll see something like this:

   # You can have the root of your site routed by hooking up ''
   # -- just remember to delete public/index.html.
   map.connect '', :controller => "home"

I think the default controller is named 'welcome' and it's all commented out, but there you go.