Drupal is a Content Management System. It is opensource . Developed in
PHP. Instead of working for hours developing those web pages and
frustating your mind (when you r not a web developer) your can
peacefully deploy drupal framework and let it manage everything. You
only need to tweak it to your needs. Just like you hav used
Wordpress .
For deploying drupal you need a web server (most probably apache) ,
mysql and php. It can save lot of your work while you just concentrate
on your business rather than developing those web pages.
If you are a web developer (maybe not true in your case) than you can
contribute to the code . It has many features that still needs
development.
http://www.drupal.org