In semiconductor quantum wells embedded in a high finesse microcavity, excitons, which are bound electron-hole systems, strongly couple to photons, forming mixed quasi-particles called polaritons. Polaritons can be coherently excited by an incident laser field and detected through the emitted light. Thanks to their excitonic component, polaritons have binary interactions which can modify their dispersion curve. These properties have allowed us to demonstrate nonlinear and quantum optical effects in the microcavity emission, and more recently quantum fluid properties. A semiconductor microcavity polariton fluid, injected by a nearly-resonant continuous wave pump laser can exhibit collective excitations that deeply modify its propagation. Superfluid behavior of the polariton fluid is obtained and manifests itself as the suppression of scattering from defects. In other conditions Cerenkov-like patterns are observed. Microcavity polaritons are thus very good tools for exploring the physics of non-equilibrium quantum fluids.