  We review the capabilities of the NASA/IPAC Extragalactic Database (NED,
http://ned.ipac.caltech.edu) for information retrieval and knowledge discovery
in the context of a globally distributed virtual observatory. Since its
inception in 1990, NED has provided astronomers world-wide with the results of
a systematic cross-correlation of catalogs covering all wavelengths, along with
thousands of extragalactic observations culled from published journal articles.
NED is continuously being expanded and revised to include new catalogs and
published observations, each undergoing a process of cross-identification to
capture the current state of knowledge about extragalactic sources in a
panchromatic fashion. In addition to assimilating data from the literature, the
team is incrementally folding in millions of observations from new large-scale
sky surveys such as 2MASS, NVSS, APM, and SDSS. At the time of writing the
system contains over 3.3 million unique objects with 4.2 million
cross-identifications. We summarize the recent evolution of NED from its
initial emphasis on object name-, position-, and literature-based queries into
a research environment that also assists statistical data exploration and
discovery using large samples of objects. Newer capabilities enable "Web
mining" of entries in geographically distributed astronomical archives that are
indexed by object names and positions in NED, sample building using constraints
on redshifts and object types, and an image archive. A pilot study demonstrates
how NED is being used in conjunction with linked survey archives to
characterize the properties of galaxy classes to form a training set for
machine learning algorithms. Opportunities for tighter integration of NED
capabilities into data mining tools for astronomy archives are also discussed.
