In versions of Rails before 3.0 (including 3.0 itself), static assets lived in the public/ directory, as follows:
public/stylesheets
public/javascripts
public/images
Files in these directories are (even post-3.0) automatically served up via requests to http://example.com/stylesheets, etc.
Starting in Rails 3.1, there are three canonical directories for static assets, each with its own purpose:
app/assets: assets specific to the present application
lib/assets: assets for libraries written by your dev team
vendor/assets: assets from third-party vendors