Content metadata describes the contents of a file in human-readable terms. The ESML content metadata will contain information such as the distributors of the data products and their contact information, spatial and temporal coverage of data products, data quality, etc., and thus allow search capability.
Syntactic metadata describe the structure of the file in machine-readable terms.
A third level is semantic metadata, which describe the contents of a file in machine-readable terms such that an application can interpret the data in an intelligent manner. The ESML team is in the process of designing and developing ESML libraries that can be used by programmers and application developers. A version of the library will be posted soon. Please join the mailing list if you want to be notified of the release date.