The content of api.census.gov/data.json is based largely on the Open Project Data Common Core Metadata Schema and is extended to include metadata specific to Census Bureau datasets. The api.census.gov/data.xml URI may be used to access the same information as XML.
In addition to the above URIs dataset discovery is available for the entire vintage/dataset hierarchy as well. For example, the following URIs may be used to discover all datasets within the 2010 vintage:
The Open Project Data Common Core Metadata Schema documentation is a good starting point for understanding the fields output by the discovery service.