Content Types are the heart and soul of Sense/Net 6.0.
A content type is a reusable set of fields you want to apply to certain contents. Everything (every content) you see in Portal Explorer and on Sense/Net 6.0 portal pages is defined by Content Types - all the files, users, groups, webcontents, page templates, pages and even folders.
If you will be building a site the first thing you will be bumping into are Content Types. For example if you want to run a news site the first thing you will have to do before being able to start off with any kind of designing or programming is define what an article will contain - define its Content Type. Lets say you decide that an Article needs to have an author, title, abstract, text, and related articles. You will have to create this Content Type. From then on you will be able to start creating and managing these articles in the backend of your portal via the Portal Explorer. And only after that will you will be creating specific Content Views to display these articles on your website in the layout(s) you prefer.
If you want to dig deeper into this topic read the wiki page on Content Types and Tamás's article on Content Type Definitions. If you want to move on check how you can create a Content View to display a Content Type.