Your content has an underlying structure for what needs to be stored and how it's related.
Structured vs Unstructured Data
Unstructured content is editable HTML in a text box. It is flexible, but also brittle and may be difficult to consistently reuse styles and components across pages.
Structured content defines the fields needed for a component and provides a standard way to display it. For example, you could write "Topics: one, two, three" at the bottom of every article, or use the taxonomy system.
Think about when to allow unstructured content, and when you need to define exactly how it should work in the system.
Content Modeling
Think about all the content your site needs to support. The content model represents the structure of the content and defines how it will be implemented.
This step explores the needs of the system and experiments with different ways of designing it. The right answer depends on what the system does and how it will be used, but it will be able to store and retrieve data effectively.
Create a spreadsheet with the field values you need if you have preexisting content. This is easier to modify and discuss with clients and team members.