Structured Content is the meat and potatoes of a Content Management System such as Drupal. We use the concept of Structured Content to design pages and components.
A simple example of Structured Content is an email. The form is a container for four fields: a sender, receiver, subject and body.
When building a CMS, you start by examining the pages and identify common display patterns. A row of Cards is a good example. Most likely, you will find the cards share the same display pattern: a title, some text and possibly an icon or image. Each card is identical in layout. You would build the edit form by adding those elements. By adding style rules, you now have a basic Card Component.
Data items may be text, numbers, or symbols.
Data, without structure, is meaningless.
Data relationships are the first step in turning data into information.
Structured content is a way of grouping data items.
Data is provided a framework to become information.
Rules are provided for fields.