  This review provides a comprehensive overview of current research on the
structural, electronic, and magnetic characteristics of the recently discovered
high-temperature superconductor La$_3$Ni$_2$O$_7$ under high pressures. We
present the experimental results for synthesizing and characterizing this
material, derived from measurements of transport, thermodynamics, and various
spectroscopic techniques, and discuss their physical implications. We also
explore theoretical models proposed to describe the electronic structures and
superconducting pairing symmetry in La$_3$Ni$_2$O$_7$, highlighting the
intricate interplay between electronic correlations and magnetic interactions.
Despite these advances, challenges remain in growing high-quality samples free
of extrinsic phases and oxygen deficiencies and in developing reliable
measurement tools for determining diamagnetism and other physical quantities
under high pressures. Further investigations in these areas are essential to
deepening our understanding of the physical properties of La$_3$Ni$_2$O$_7$ and
unlocking its superconducting pairing mechanism.
