All data has some structure, but ‘structured data’ refers to data where the structural relation between elements is explicit in the way the data is stored on a computer disk. XML and JSON are common formats that allow many types of structure to be represented. The internal representation of, for example, word-processing documents or PDF documents reflects the positioning of entities on the page, not their logical structure, which is correspondingly difficult or impossible to extract automatically.
Data that resides in fixed fields within a record or file. Relational databases and spreadsheets are examples of structured data. Although data in XML files is not fixed in locations like traditional database records, it is nevertheless structured, because the data is tagged and can be accurately identified.
Source: PC Magazine encyclopaedia — http://www.pcmag.com/encyclopedia/term/52162/ , cited by EU OD