The document summarizes changes in Rails 3 including: 1. Bundler is introduced for managing gem dependencies through a Gemfile. 2. The ActiveRecord query interface is updated with method chaining and relations to unify finders, named scopes, and with_scope. 3. The ActiveRecord validation API is updated to use hashes instead of separate validator methods for validation options. 4. Views are updated with automatic XSS escaping, unobtrusive JavaScript, and consistent <%= %> usage for output. 5. Internationalization is updated with %{} instead of {{ }} for translations.