If Mongrel and lighttpd are not installed, by default Rails will use WEBrick, the webserver that ships with Ruby. All these will guarantee you to always get up and run promptly. Rails will check first if Mongrel exists when you run script/server, then check on lighttpd and would finally fall back to WEBrick which is a small Ruby web server appropriate for development but not for production. Mongrel is a Ruby-based webserver possessing a C component that requires compilation. It is suitable for development and operation of Rails application. Lighttpd is considerably faster than MOngerl and WEBrick but entails additional installation and presently only works well on OS X/Unix.