What is Drupal?
Drupal is a very power Content Management System, otherwise known as a CMS. It handles all of your website's content and allows you to choose different ways of displaying that content to your viewers. One of the biggest advantages of using Drupal is that it is an Open Source CMS, so there are no licensing costs and the software itself is free of charge. The flexibility of the Drupal as a CMS framework is what has made it so popular with developers, while the community as a whole contributes back to Drupal's overall growth.
What one might know of as a Plugin in Wordpress, is actually called a Module in Drupal. Drupal modules extend Drupal's core functionality in every direction possible. Whether you are looking to build a professional business website, social networking community, an e-commerce store, a multimedia driven website, or just a personal blog, there is probably a module that does the job. The best part is, if there isn't, you can code one yourself if you know PHP, or you can find someone who does custom Drupal module development and hire them to do it for you.
There are also some great Drupal themes being made these days. Originally, many of Drupal themes were very dull and almost too mechanical for anyone to want to use the default or most contributed themes, but that was mainly due to the fact that it was still a hard CMS to theme back in Drupal 5. Advancements in the core technology of Drupal 6 have allowed for Drupal 6 to introduce a whole new set of themers and therefore more quality Drupal 6 themes to the Drupal community. Many of these new Drupal 6 themes are more reminiscent to what you might see on a Wordpress website, so things are definitely coming around in the design world of Drupal as well as the Drupal website development world.
The Drupal 7 release has has it's fair share of problems, but one thing is for certain, Drupal 7 themes have the potential to be amazing with the new advancements in Drupal core in the theme layer. Building upon the advancements in Drupal 6, Drupal 7 allows themers and even non-themers to harness the power of a Drupal theme to it's fullest. Modules like the Sweaver module allow those with absolutely no Drupal theming experience to dive right into a Drupal theme from the Drupal API and start changing it to suit your needs. If that isn't powerful, I am not sure what is. In Drupal 6, the most powerful theme related tool in the Drupal UI was probably the CSS Injector module.
It is quite evident that there are many reasons to why you should use Drupal, but you don't have to take my word for it, visit Drupal.org and hang out for a bit. Take a look at some of the demonstration videos and demo website available, along with checking out some of the modules and how they function through their demo sites and you make the decision for yourself. You should quickly catch on to why Drupal is definitely my CMS of choice.