They represent the progress of the overall reaction at a molecular level. The sequence of elementary steps that lead to product formation is called the reaction mechanism. Each elementary step will have a molecularity . The molecularity is the number of molecules that react in an elementary process. Unimolecular steps are not as common as Bimolecular steps.
Radioisotopes decay via a unimolecular elementary step.
Bimolecular steps involve the collision of two reacting molecules. This is the most common elementary step. Termolecular steps involve the simultaneous collision of three reacting species. This is a very rare occurance. It is important to distinguish between molecularity and order.