Supersymmetry
Generally speaking, supersymmetry, or SUSY for short, treats matter particles (fermions) and force particles and Higgs (bosons) in a unified framework. Formally, for each SM particle a superpartner is predicted. Partners and superpartners have the same fundamental properties, except that of spin, which differs by 1/2, and mass, as supersymmetric partners of electrons or other SM particles have not been observed yet, and must therefore be much heavier.
According to the current understanding, supersymmetry is a key ingredient in the quest of unifying the fundamental forces. Grand unification theories, GUTs, which describe leptons and quarks in the same formalism, seem to need SUSY to unify the electromagnetic, weak and strong forces. This is expected to happen at incredibly high energies, ~ 1016 GeV. Supersymmetry is required in string theory - making it superstring theory - in order to achieve superunification of all fundamental forces of nature, including gravity.
SUSY proposes good candidates for dark matter observed in the Universe. The Lightest Supersymmetric particle (LSP) can be the lightest neutralino, the common super partner of the photon, Z boson and neutral Higgs bosons, or the gravitino, the superpartner of the graviton.