From USFWS service description: "This data service identifies, in general, the areas where final critical habitat exist for species that are endangered or threatened. Special protections and/or restrictions are possible in areas where federal funding, permits, licenses, authorizations or actions occur or are required. Critical habitat are areas considered essential for the conservation of a listed species. These areas provide notice to the public and land managers the importance of these areas to the conservation of the species."
The data can be downloaded in a shapefile, viewed on the Fish and Wildlife Service's map viewer, or accessed through a web service. It's also possible to view the data through the ScienceBase interactive mapper and the National Map Viewer (see bar directly above inset map).