Wordpress is a Content Management System (CMS), but what is a CMS?
The best definition of CMS is the one from Wikipedia which is more business oriented but it is applying to any environment or purpose or level of your sites and skills:
"A content management system (CMS) is the collection of procedures used to manage work flow in a collaborative environment...
The procedures are designed to do the following:

• Allow for a large number of people to contribute to and share stored data
• Control access to data, based on user roles
• Aid in easy storage and retrieval of data
• Reduce repetitive duplicate input
• Improve the ease of report writing
• Improve communication between users

In a CMS, data can be defined as nearly anything: documents, movies, pictures, phone numbers, scientific data, and so forth. CMSes are frequently used for storing, controlling, revising, semantically enriching, and publishing documentation."
It is also a well defined  Enterprise Content Management System (ECM) with main purpose being:

• to streamline access
• eliminate bottlenecks
• optimize security and
• maintain integrity 

Those definitions are created on purpose to cover a big area, but I am a believer on understanding the big picture so  I'll continue to elaborate on this direction.

Source : http://wizzley.com/wordpress-as-content-management-system/