  In this review we discuss intriguing properties of apparently classical
optical fields, that go beyond purely classical context and allow us to speak
about quantum characteristics of such fields and about their applications in
quantum technologies. We briefly define the genuinely quantum concepts of
entanglement and steering. We then move to the boarder line between classical
and quantum world introducing quantum discord, a more general concept of
quantum coherence, and finally a controversial notion of classical
entanglement. To unveil the quantum aspects of often classically perceived
systems, we focus more in detail on quantum discordant correlations between the
light modes and on nonseparability properties of optical vector fields leading
to entanglement between different degrees of freedom of a single beam. To
illustrate the aptitude of different types of correlated systems to act as
quantum or quantum-like resource, entanglement activation from discord,
high-precision measurements with classical entanglement and quantum information
tasks using intra-system correlations are discussed. The common themes behind
the versatile quantum properties of seemingly classical light are coherence,
polarization and inter and intra--mode quantum correlations.
