The Data Format Description Language (DFDL) is a specification, developed by the Open Grid Forum, and also published as ISO/IEC 23415:2004. DFDL is capable of describing many data formats, including both textual and binary, scientific and numeric, legacy and modern, commercial record-oriented, and many industry and military standards. It defines a language that is a subset of W3C XML schema to describe the logical format of the data, and annotations within the schema to describe the physical representation.

Daffodil is an open source implementation of the DFDL specification that uses these DFDL schemas to parse fixed format data into an infoset, which is most commonly represented as either XML or JSON. This allows the use of well-established XML or JSON technologies and libraries to consume, inspect, and manipulate fixed format data in existing solutions. Daffodil is also capable of the reverse by serializing or "unparsing" an XML or JSON infoset back to the original data format.