A catalyst is a substance that speeds up a chemical reaction without being consumed in the process. It lowers the activation energy required for the reaction to occur, allowing it to happen more quickly or at lower temperatures. Catalysts are essential in various industries, including petrochemicals and pharmaceuticals, where they help produce products efficiently. There are two main types of catalysts: homogeneous and heterogeneous. Homogeneous catalysts are in the same phase as the reactants, while heterogeneous catalysts exist in a different phase. Both types play crucial roles in improving reaction rates and are widely used in scientific research and industrial applications.