If you're encountering an error such as:
ActionView::Template::Error (application.css isn't precompiled)
then you are probably using Rails 3.1 and have forgotten to run the following rake task:
RAILS_ENV=production bundle exec rake assets:precompile.
The Asset Pipeline, that's how this nice feature in Rails is called, is a very mighty concept so you possibly want to have a look at the following Rails guide to get a deeper understanding of the topic: