Content Types are really the heart of how Drupal stores and organizes information. A content type is a definition of a kind of information that a site can store. For example, you can have a content type that defines what a "Job Posting" is like, or what a "Blog Entry" is like, or what a "Hotel Room" contains, and so on.
Prior to interviewing with BYU's Computer Science Department for a job as a web developer, I hadn't ever heard of Drupal before. I'd used Wordpress, which I thought was decent (if somewhat inflexible), and I'd only heard of Joomla or Mambo in passing. Drupal was a new beast for me.
As soon as I was hired, I had to plunge straight in. I should warn you, I'm a programmer at heart, so I took to Drupal very quickly. There are still some things that flummox me, but I feel that I understand the system.
Well, I've done it again. I've gone and changed the underlying content management system that I use on my site. I've gone with the CMS Drupal, which I've come to know quite intimately ever since beginning work for BYU's Computer Science Department.