Rails 3/Ruby on Rails framework< Rails 3
What's a web framework?Edit
For ages people have programmed the web with Perl and PHP. These languages did not have a definite pattern. People could do any thing with them: put code in middle of web pages, connect to database from anywhere, put images into any folder or insert them into web pages, and so on. All this created a messy, lengthy code which, when written by one programmer was quiet unreadable by another. Even the programmer who wrote such code might not be able to decipher after returning to it after some months.
Many open source projects emerged. They all promised and mostly delivered a good web development experience. For example Python language got Django and Turbogears. PHP got Symfony, Codeigniter and PHP on Tracks. Ruby got Merb, and of course, Rails.
Out of these frameworks, Ruby on Rails, or Rails for short, has shot into popularity. Why so? Read the book and you will know.