Supergravity is a class of local supersymmetry theories. Like globally supersymmetry theories, it associates a fermion and a boson to form a multiplet of particles. When global supersymmetry is 'gauged' to make a it local symmetry, supergravity is automatically obtained. The simplest theory is in 4 dimensions with a single supersymmetry generator. The graviton, the quantum of gravitation, is then associated with a fermion of spin 3/2 called a gravitino, and matter and geometry are then unified.
Supergravity can be formulated in several dimensions and with a maximum of 8 supersymmetry generators. It can potentially unite all the interactions and particles by space-time 'supergeometry' (but this has yet to be proved). M theory appears to have it as an approximation to low energy.