From the Ruby on Rails website:
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.
Also see our Rails 3.1 self-guided tour and the Ruby topic.
Self-guided Ruby on Rails 3.1 Tutorial — November 4, 2011
Learn Ruby on Rails by following commits in an example application
HAML users repent before it's too late — December 1, 2011
While HAML makes it easier to write HTML, reading HAML is a different story
Rails 4.0 will drop support for Ruby 1.8.7 — February 2, 2012
If you're still stuck on Ruby 1.8 the pressure is on to upgrade
Ruby/Rails Compatibility Matrix — March 14, 2012
Which versions of Ruby are supported by which versions of Rails