Drupal Gardens is a software as a service (Saas) offering from Acquia. There is nothing to install, simply create an account, login and begin creating a Drupal site. Drupal Gardens offers an almost zero degree entry point and is ideal for those evaluating Drupal, you can jump in and get a feel for Drupal immediately, without any concern or effort of setting up a LAMP stack, or maintaining a system. It also means you may use what ever local enviroment/OS you want. Acquia has been steadily advancing the Gardens platform, and has added in a nice selection of Drupal features. The down side of Gardens would be that a user is limited to using the modules that are provided, there is no loading additional modules. In addition to the core Drupal modules, Garden's provides 28 additional contrib modules (as of June 2012).
While you are fairly locked in to the features you are given, you aren't locked into DrupalGardens. Even though the modules are limited, they are a robust cross section of the most common Drupal modules, and offer quite a range of capabilities. Additionally, Drupal Gardens doesn't tie you down to their platform, like most SaaS offerings. If you out grow Drupal Gardens, your site can be exported and imported into a traditional Drupal install and then developed as normal Drupal. Horaah for Open Source!
Using Drupal Gardens