Sinatra is a Ruby DSL or framework for creating web applications. It is intended to be simple and fast to work with, as a result your application can have only one Ruby file. The following code is placed on their homepage to showcase how easy and simple it could be to create a running Sinatra application:
get '/' do
Sinatra is a fast and effortless way to create a mockup, an API server or even could be used to create production applications. It is Rack based and modular. Out of the box, it supports many templating engines.