16. A computer-readable storage device medium having stored thereon a data structure defining a platform-independent data application for implementation through an application program executing on a computer, the data structure comprising: a first data type storing a platform-independent definition of a data schema for defining data types in a markup language; a second data type storing a platform-independent definition of a user interface for accessing data stored in the data types in the markup language; a third data type storing a platform-independent definition of business logic for enforcing validation rules and actions associated with the data types in the markup language; and a fourth data type storing a platform-independent definition of application information for providing a description of the platform-independent data application, wherein the application information comprises meta-information about the platform-independent data application in the markup language; wherein the platform-independent data application is stored in a markup language file defined in the markup language and has a platform-independent format that includes the first, second, third, and fourth data types, the markup language file being executable within an application program from the definitions of the data schema, user interface, and business logic in the markup language file.
17. The computer-readable storage device of claim 16 , wherein the platform-independent definition of the data schema comprises a platform-independent definition of a structure of the data types and a platform-independent definition of relationships between the data types.