Big Data is a technology developed for 3-V management of data by which large volumes and different varieties of data would be processed in optimal velocity. The data to be dealt with may be structured or unstructured. Relational databases (spreadsheets) are typical examples of structured data and the methods, as well as the techniques for researches of relational database management are well-known. In this paper, we describe a formalism, by which, structured data, can be considered as a directly generalized model of relational databases. A higher leveled structured data, in our generalization, are defined recursively as a set or a queue of lower leveled structured data. Consequently, our study proves that many concepts and results of relational database management can be transferred to structured data, accordingly to this generalization. The sub data, the components of structured data, the functional dependencies between structured data, as well as the keys data in structured data are defined and studied. Alternately, some concepts that are defined here for structured data can be applied for relational databases, as a special case. In this paper, some operations on structured data and the homomorphism between structured data are defined and studied that appear to be quite suitable for relational databases. In fact, the formalization introduced here, offers effective methods for further structural, algebraic researches of structured data.