Post-Keynesian economics treats the economy as a monetary phenomenon, with output, employment and investment driven by expected demand by consumers. Full employment and full capacity utilisation are not guaranteed. Uncertainty and lack of information dominate decision making. Economic behaviour is social and institutional, and not necessarily rational. Unlike New Keynesianism, it rejects equilibrium and constrained optimisation of utility or profits as organising principles. Money is endogenous to the economic system and outcomes are path-dependent.
Excellent summary of different schools is by the Society for Pluralist Economics
Post-Keynesian reading list