Semi-structured data is a form of structured data that does not conform with the formal structure of data models associated with relational databases or other forms of data tables, but nonetheless contains tags or other markers to separate semantic elements and enforce hierarchies of records and fields within the data. Therefore, it is also known as schemaless or self-describing structure.
In the semi-structured data, the entities belonging to the same class may have different attributes even though they are grouped together, and the attributes' order is not important.
Semi-structured data is increasingly occurring since the advent of the Internet where full-text documents and databases are not the only forms of data any more and different applications need a medium for exchanging information. In object-oriented databases, one often finds semi-structured data.
Types of Semi-structured data
Some types of data described here as "semi-structured", especially XML, suffer from the impression that they are incapable of structural rigor at the same functional level as Relational Tables and Rows. Indeed, the view of XML as inherently semi-structured (previously, it was referred to as "unstructured") has handicapped its use for a widening range of data-centric applications. Even documents, normally thought of as the epitome of semi-structure, can be designed with virtually the same rigor as database schema, enforced by the XML schema and processed by both commercial and custom software programs without reducing their usability by human readers.
In view of this fact, XML might be referred to as having "flexible structure" capable of human-centric flow and hierarchy as well as highly rigorous element structure and data typing.
- Tutorial on semi-structured data by Peter Buneman from Symposium on Principles of Database Systems, 1997 
- The Penn database group has semi-structured and XML data project
- Stanford Universities Lore DBMS