SDSS consists of two publicly accessible components, the Catalog Archive Server, or CAS, and the Data Archive Server, or DAS.
CAS: Microsoft SQL Server 2003 32-bit database of catalog data most commonly accessible via the SkyServer
Other interfaces to the data are available as well for dedicated researchers.DAS: Flat files of images and spectra in FITS format corresponding to object coordinates (known-item search).
Access to SDSS data was also available in the initial incarnation of Galaxy Zoo, which now uses data from the Hubble Space Telescope.
A Flight Through the Universe, by the Sloan Digital Sky Survey