  Entanglement, according to Erwin Schroedinger the essence of quantum
mechanics, is at the heart of the Einstein-Podolsky-Rosen paradox and of the so
called quantum-nonlocality - the fact that a local realistic explanation of
quantum mechanics is not possible as quantitatively expressed by violation of
Bell's inequalities. Even as entanglement gains increasing importance in most
quantum information processing protocols, its conceptual foundation is still
widely debated. Among the open questions are: What is the conceptual meaning of
quantum entanglement? What are the most general constraints imposed by local
realism? Which general quantum states violate these constraints? Developing
Schroedinger's ideas in an information-theoretic context we suggest that a
natural understanding of quantum entanglement results when one accepts (1) that
the amount of information per elementary system is finite and (2) that the
information in a composite system resides more in the correlations than in
properties of individuals. The quantitative formulation of these ideas leads to
a rather natural criterion of quantum entanglement. Independently, extending
Bell's original ideas, we obtain a single general Bell inequality that
summarizes all possible constraints imposed by local realism on the
correlations for a multi-particle system. Violation of the general Bell
inequality results in an independent general criterion for quantum
entanglement. Most importantly, the two criteria agree in essence, though the
two approaches are conceptually very different. This concurrence strongly
supports the information-theoretic interpretation of quantum entanglement and
of quantum physics in general.
