First things first, all data is more or less structured. That being said, there is…

• Structured Data
• Semi-Structured Data
• Unstructured Data

I tend to think of it as: data, composite or simple, with or without content. In that context, email is structured composite data (from, to, subject, date) with unstructured content (message body). The composite data is structured. The content is unstructured. Though simple data may or may not be structured. The ‘subject’ data is unstructured. The ‘to’ data is structured. It is composed of a local-part (username) and a domain.

While content is unstructured, there may be an implied structure.

So what is the difference between structured data, semi-structured data, and unstructured data?

Structured Data

The structure is externally enforced. Data

• The data is stored in a database.
  • Relational
    • Transaction
  • XML
    • Catalog

The data itself is not structured. The structure is defined by the database. The data would be semi-structured if it was exported and transformed into JSON or XML.

Semi-Structured Data

The structure is self defined. Data and / or Content

• The data is stored as text.
  • JSON
    • User Profile
  • XML
    • Application / Form

Unstructured Data

The structure of the data is externally defined. Metadata and Content

• The data is stored in a binary format and / or document.
  • Media (e.g. Ogg).
  • Video (e.g. Vorbis).
  • Audio (e.g. Theora).
  • Image (e.g. PNG).
  • Microsoft Word
  • Adobe PDF

The structure is defined by the file format. The data is composed of structured metadata and unstructured content. That being said, a video is composed of frames and an image is composed of pixels.

• The data is stored as plain text.
  • Log

The structure is defined by a pattern in the logging configuration file. The data is composed of structured metadata (e.g. severity) and unstructured content (log message).

• The data is user generated.
  • Status (Facebook)
  • Tweet (Twitter)
  • Comment (WordPress)

The structure is defined by the application / form. The data is composed of structured metadata (e.g. user ID) and unstructured content (user message).

Update

I would say that the structure of content is user defined and thus interpreted. However, content is often a component of data (unstructured, externally defined). Though if I typed up this post in gedit and saved it as a text file, that might constitute content independent of data.

With all all that said, the structure of data is not exactly black and white.