  A review of the phenomenology and microscopy of cuprate superconductors is
presented, with particular attention to universal conductance features, which
reveal the existence of two electronic subsystems. The overall electronic
system consists of $1+p$ charges, where $p$ is the doping. At low dopings
exactly one hole is localized per planar copper-oxygen unit, while upon
increasing doping and temperature, the hole is gradually delocalized and
becomes itinerant. Remarkably, the itinerant holes exhibit identical
Fermi-liquid character across the cuprate phase diagram. This universality
enables a simple count of carrier density and yields comprehensive
understanding of the key features in the normal and superconducting state. A
possible superconducting mechanism is presented, compatible with the key
experimental facts. The base of this mechanism is the interaction of fast
Fermi-liquid carriers with localized holes. A change in the microscopic nature
of chemical bonding in the copper-oxide planes, from ionic to covalent, is
invoked to explain the phase diagram of these fascinating compounds.
