Provides a minimal, modular interface between web servers that suport Ruby and Ruby frameworks. It is a layer of abstraction above the web server implementation (allowing interfacing with a Ruby app regardless of what web server and app server it's running on). It interfaces the Net::HTTP Ruby library.
Rails uses Rack. rails server does the basic job of creating Rack::Server object starting the webserver.
Implementation:
Rails::Server.new.tap do |server|
require APP_PATH
Dir.chdir(Rails.application.root)
server.start
end