Drupal is a CMS system that can be freely downloaded and used from the Drupal website. It is not an object orientated PHP driven system and so everything is available through function calls and variables.
I have heard that the learning curve on developing Drupal is quite steep, but once you get around this initial hurdle the whole thing is pretty simple. This ease of development means that it is possible to take an idea and use Drupal and all its features as the CMS behind your idea, thus saving you time on your project development.
Drupal works in a kind of framework pattern in that you need to create files and folders in order to extend or overwrite the existing functionality, which is already quite extensive. This can trip up a lot of developers who are new to the system. My recommendation is to get someone to explain the initial steps to you and and then try it out for yourself. However, there is plenty of documentation available on the Drupal website.
From visiting the Drupal website to having a working version of the system on a test server took me literally 10 minutes, including the time it took to download and upload the files. I have used Wordpress extensively in the past, but have always found that it can only be customized to a certain extent before you need to start hacking the source code. Although this is getting better with newer versions of Worpress I will be using Drual in my next website build.