If you write much Ruby, you’ve definitely encountered Rack. For users of frameworks like Rails and Sinatra, Rack can easily be relegated to the world of “plumbing” down there with the web and app servers; something to be configured and ignored. But that would be a mistake. This intro to Rack provides the most straightforward explanation of Rack I’ve seen. Every concept from the bottom up is discussed with clear code examples. If you write web applications using Ruby, you owe it to yourself to brush up on your Rack knowledge.
From RubyLearning.com and Satish Talim: A Quick Introduction to Rack