NED is built around a master list of extragalactic objects for which cross-identifications of names have been established, accurate positions and redshifts entered to the extent possible, and some basic data collected. Bibliographic references relevant to individual objects have been compiled, and abstracts of extragalactic interest are kept on line. Detailed and referenced photometry, position, and redshift data, have been taken from large compilations and from the literature.
NED also includes images from 2MASS, from the literature, and from the Digitized Sky Survey.
As of March 2014, NED contains 206 million distinct astronomical objects with 232 million cross-identifications across multiple wavelengths, with redshift measurements for 5 million objects, 1.9 billion photometric data points, 609 million diameter measurements, 71 thousand redshift-independent distances for over 15 thousand galaxies, 310 thousand detailed classifications for 230 thousand objects, and 2.6 million images, maps and external links, together with links to 65 thousand journal articles, notes and abstracts.