structured data sources

Well it's a rather nebulous term but I'd define structured data as any data that can be represented in terms of records and fields and relations between them. In other words, it's data which has some sort of schema and so can be easily represented by a relational database.

In contrast unstructured data is any other kind of data such as notes, documents or even hierarchical data even though that clearly does have some sort of structure.

The definition of unstructured data in Wikipedia (http://en.wikipedia.org/wiki/Unstructured_data) may be helpful in this regard.