Content types are types created by your organization. They can describe a document type or list item. A content type has its own workflow and specific metadata. These types can be reused throughout the entire site and are managed at site level.
For example: a financial form, it has a workflow that sends an email to the financial assistant, and has extra fields like department, total cost … When somebody creates a document of this type; it inherits these properties and activates the workflow.
You can still add extra metadata or specific workflows on items.