Metadata.

Content is identified with metadata. Important forms of metadata include:

• Structural metadata relating to the structure of the content. This is specified by the components and elements used within content types.
• Administrative metadata relating to the history of content items, such as who created the content and when it was published or revised.
• Technical metadata relating to the characteristics of content formats, such as the size of images.
• Descriptive metadata that identifies what topics or themes the information relates to. 

All forms of metadata can be used both internally to support content operations, and externally to support content delivery. Descriptive metadata plays an especially important role, and is closely associated with taxonomies used to categorize content. Descriptive metadata also supports content discovery by audiences. The most important standards for descriptive metadata are schema.org for search and voice applications, Open Graph for social media, and Twitter metadata.