  Sky surveys represent a fundamental data basis for astronomy. We use them to
map in a systematic way the universe and its constituents, and to discover new
types of objects or phenomena. We review the subject, with an emphasis on the
wide-field imaging surveys, placing them in a broader scientific and historical
context. Surveys are the largest data generators in astronomy, propelled by the
advances in information and computation technology, and have transformed the
ways in which astronomy is done. We describe the variety and the general
properties of surveys, the ways in which they may be quantified and compared,
and offer some figures of merit that can be used to compare their scientific
discovery potential. Surveys enable a very wide range of science; that is
perhaps their key unifying characteristic. As new domains of the observable
parameter space open up thanks to the advances in technology, surveys are often
the initial step in their exploration. Science can be done with the survey data
alone or a combination of different surveys, or with a targeted follow-up of
potentially interesting selected sources. Surveys can be used to generate
large, statistical samples of objects that can be studied as populations, or as
tracers of larger structures. They can be also used to discover or generate
samples of rare or unusual objects, and may lead to discoveries of some
previously unknown types. We discuss a general framework of parameter spaces
that can be used for an assessment and comparison of different surveys, and the
strategies for their scientific exploration. As we move into the Petascale
regime, an effective processing and scientific exploitation of such large data
sets and data streams poses many challenges, some of which may be addressed in
the framework of Virtual Observatory and Astroinformatics, with a broader
application of data mining and knowledge discovery technologies.
