The catalytic converter is located between the exhaust manifold(s) and muffler. It looks similar to a muffler and may have a heat shield.
Three-Way Converter (TWC). Most vehicles use a TWC. Oxygen sensors are required to regulate the air-fuel ratio close to stoichiometric (14.7 to 1) for it to efficiently reduce emissions.
Catalyst. A TWC contains a catalyst that when very hot (over 500°F) accelerates the chemical change of CO, HC, and NOX into water vapor (H2O), CO2, and nitrogen (N).
Pre-Catalyst. Engines that burn ultra lean, like GDI engines, may also need a smaller pre-catalyst (warm up converter). It is placed closer to the exhaust manifold, heating up to operating temperature faster, to better reduce NOX.