Rack is a specification implemented by not only Sinatra, but also Rails, Merb, Ramaze, and a number of other Ruby projects. It’s an extremely simple protocol specifying how an HTTP server (such as Thin, which we’ve used throughout the book) interfaces with an application object, like Sinatra::Application, without having to know anything about Sinatra in particular. In short, Rack defines the higher-level vocabulary that hardware and software can use to communicate. The Rack homepage, http://rack.rubyforge.org, is shown in Figure 3-4.