API

HIU has a distributed infrastucture so there is no one domain for APIs. GeoNode, a core part of HIU's infrastructure, supports 3 APIs: (1) CSW, (2) OGC, and (3) internal GeoNode API. Each HIU instance of GeoNode can be interfaced using all of those APIs.
If you are starting off, we'd suggest you take a look at State GeoNode's API (http://geonode.state.gov/about/api/).