Both WordPress and Drupal are open source CMS that accomplish the same thing in the end. However, there are some major differences between them. 

WordPress evolved from a blogging platform. It is a solution for websites managed by one person. WordPress is the simplest CMS to use. However it's popularity makes it a target for hacker attacks so you’ll need regular backups and non-mainstream plugins.

Drupal is the most flexible content management system. Within Drupal you can define your own data structures and relationships beyond the articles, users etc. It is a very comprehensive CMS. It’s functionality can be extended with the help of modules and themes. However, they are much harder to write than for Wordpress because of the complexity of the system. If you need to implement something unique on your platform, Drupal may work.