  Quantum steering, also called Einstein-Podolsky-Rosen steering, is the
intriguing phenomenon associated with the ability of spatially separated
observers to steer---by means of local measurements---the set of conditional
quantum states accessible by a distant party. In the light of quantum
information, all steerable quantum states are known to be resources for quantum
information processing tasks. Here, via a quantity dubbed steering fraction, we
derive a simple, but general criterion that allows one to identify quantum
states that can exhibit quantum steering, thus making an important step towards
the characterization of steerable quantum states. The criterion, in turn, also
provides upper bounds on the largest steering-inequality violation achievable
by arbitrary finite-dimensional maximally entangled states. For the
quantification of steerability, we prove that a strengthened version of the
steering fraction is a convex steering monotone and demonstrate how it is
related to two others, namely, steerable weight and steering robustness. Using
these tools, we further demonstrate the superactivation of steerability, i.e.,
we show how the steerability of certain entangled, but unsteerable quantum
states can be recovered by allowing joint measurements on multiple copies of
the same state. In particular, our approach allows one to explicitly construct
a steering inequality to manifest this phenomenon. Finally, we prove that there
exist quantum states (including some unsteerable under projective measurements)
whose steering-inequality violation can be arbitrarily amplified by allowing
joint measurements on three copies of the same state. For completeness, we also
demonstrate how the largest steering-inequality violation can be used to bound
the largest Bell-inequality violation and derive, analogously, a simple
sufficient condition for Bell-nonlocality from the latter.
