Ruby on Rails is a full-stack web framework for building web applications using the Ruby programming language. It provides tools for rapid website development including common CRUD (create, read, update, delete) functionality. While initially popular, Ruby on Rails has faced some criticisms including that it is not truly a framework or language. There are multiple virtual machines that can run Ruby code including MRI, JRuby, and IronRuby. Ruby on Rails applications can be easily deployed on platforms like Apache, Heroku, and Google App Engine.