  Fluctuations of conserved quantities, such as baryon, electric charge and
strangeness number, are sensitive observables in relativistic heavy-ion
collisions to probe the QCD phase transition and search for the QCD critical
point. In this paper, we review the experimental measurements of the cumulants
(up to fourth order) of event-by-event net-proton (proxy for net-baryon),
net-charge and net-kaon (proxy for net-strangeness) multiplicity distributions
in Au+Au collisions at $\sqrt{s_{NN}}=7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, 200$
GeV from the first phase of beam energy scan program at the Relativistic
Heavy-Ion Collider (RHIC). We also summarize the data analysis methods of
suppressing the volume fluctuations, auto-correlations and the unified
description of efficiency correction and error estimation. Based on theoretical
and model calculations, we will discuss the characteristic signatures of
critical point as well as backgrounds for the fluctuation observables in
heavy-ion collisions. The physics implications and the future second phase of
the beam energy scan (2019-2020) at RHIC will be also discussed.
