  High-$T_c$ cuprates differ from conventional superconductors in three crucial
aspects: the superconducting state descends from a strongly correlated
Mott-Hubbard insulator, the order parameter exhibits d-wave symmetry and
superconducting fluctuations play an all important role. We formulate a theory
of the pseudogap state in the cuprates by taking the advantage of these unusual
features. The effective low energy theory within the pseudogap phase is shown
to be equivalent to the (anisotropic) quantum electrodynamics in (2+1)
space-time dimensions (QED$_3$). The role of Dirac fermions is played by the
nodal BdG quasiparticles while the massless gauge field arises through
unbinding of quantum vortex-antivortex degrees of freedom. A detailed
derivation of this QED$_3$ theory is given and some of its main physical
consequences are inferred for the pseudogap state. We focus on the properties
of symmetric QED$_3$ and propose that inside the pairing protectorate it
assumes the role reminiscent of that played by the Fermi liquid theory in
conventional metals.
