A content management system is made for managing content - who would have thought?!
So the next question is: What is content?
In media production and publishing, content is information and experiences that may provide value for an end-user/audience in specific contexts (Wikipedia).
Ok, I see, but what is it that provides value?
This question is indeed very hard to answer and quite individual but I think you now get the idea of content and a content management system like Joomla!.
A content management system is useful for managing information that provides value for your audience (Hagen Graf :-) ).
Types of content
In core Joomla! you have articles, categories, web links, banners, contacts and feeds. I am not sure whether creating value with banners is possible but banners are also a kind of content. Modules can be used for content creation as well. Often you may want to enrich your content with files like photos and other media types. For this Joomla! has its Media manager. This is what Joomla! core provides as your toolbox to manage all these bits and pieces to create value to your audience. I am going to cover these tools in the next chapters.