A Content Management System (CMS) website is built of pages or other entities which contain ‘fields’ of information. Pages which share a common set of fields are a common Content Type; perhaps an Article or a Basic page type in Drupal. The pages are accessed with a menu which may pull a single page or which may retrieve a View that lists or presents pages of a common type, for example a whole set of Articles on a given topic. Figuring out what page or content types, what fields belong on those pages, and then what Views and menus will make access user-friendly is what you will do in the design phase of building your website.
Especially for what fields will be used and what they will be called, you are really working on the Schema or Architecture of the data structure that will underlie your website. Don’t freakout about it, as Drupal does all the technical whiz-bang part of that for you. In fact, that is what makes it so extraordinary.
That said, it can help to understand a little about the principles and value of organizing the architecture of your site; especially if it is likely to be of some level of detail and size because you think big.
A fellow names Jay Rockowitz outlines the value of doing this.
It is really brought to light with many different content types being used in a site. The Type Tray module’s way of categorizing and organizing these in a nice visual way is helpful to understand.
Mr. Rockowitz moves deeper into schema in this documentation but he doesn’t just hang you out in the entity diagram world; rather, he does a nice job in showing links to specific modules that are helpful with things like field type management and use in applications.