The web application that we'll be building throughout the course of the book will be written in Ruby using the open source Rails framework. To quote http://rubyonrails.org:
"Ruby on Rails® is an open-source web framework that's optimized for programmer happiness and sustainable productivity. It lets you write beautiful code by favoring convention over configuration."
I couldn't agree more. I find Rails fun to use and the development process to be very intuitive. The community is very active and there are tons of free resources online. It's also worth mentioning that Shopify is written in Rails and many popular Ruby libraries such as Active Merchant and Liquid are extractions of the Shopify source code.