Andrew Lynch posted a fantastic list of 32 Web Frameworks over at memeburn.com. He starts off that list with Ruby on Rails, Django, Drupal, CakePHP...well you get the idea.
Frameworks are built on top of programming languages and provide methods of streamlining some of the more mundane and common tasks associated with web development. In essence, they allow developers to achieve more with less coding, saving both time and money. They provide a range of simple methods that help you connect to a database, authenticate users and build an admin backend, all with a few lines of code.
Be sure to read the complete story so you can see Andrew's review of 32 web frameworks that you have to choose from for your next project.