Calabash could be compared to Selenium WebDriver. However, it is important to realize that interacting with a web app from a desktop computer is vastly different than interacting with a native app using a touch screen. it provides APIs that are specialized to native apps running on touch screen devices.(We are thankful to Vishvas..Who has taken ownership for this tutorial).
Those who are keen to learn basics of calabash with ruby then they might see the only course on internet that has a good coverage.
Or if you want to learn it gradually and completely and want to learn by doing then official site is the best place to start with.