The mechanism of a chemical reaction is typically written down as a series of elementary steps. The steps themselves are characterized by their "molecularity". The molecularity is a way of stating exactly how many molecules are involved in the elementary step. One molecule reacting by itself is a unimolecular reaction, or you say that the molecularity is one. Two molecules reacting is called a bimolecular step (the molecularity is two). Generally speaking only uni- and bimolecular steps are proposed in reaction mechanisms.
The sum of these steps is the overall reaction.
These steps detail the means by which the reactant molecules transform into the product molecules.
It is possible that the overall reaction is in fact a reflection of the elementary steps. For example, imagine a reaction that proceeds by two molecules colliding and then forming the products.
\[\rm{NOBr +NOBr \rightarrow 2NO + Br_2}\]
In this case, the overall reaction is the same as the only elementary step in the mechanism.
Alternatively, the reaction might occur through a number of elementary steps that occur along the way. For example, we might have a reaction such as
\[\rm{NO_2 + CO \rightarrow NO + CO_2}\]
It can be shown that the rate law for this reaction is rate = k[NO2]2. The mechanism for this reaction has two elementary steps
\[\rm{step 1 \hskip 8pt NO_2 + NO_2 \rightarrow NO_3 + NO}\]
\[\rm{step 2 \hskip 8pt NO_3 + CO \rightarrow NO_2 + CO_2}\]
Rather than an NO2 molecule colliding with a CO and exchanging an oxygen atom, the first step of this mechanism is the collision of two NO2 molecules that will react with each other to form NO and NO3. Subsequently the NO3 collides with the CO to exchange the oxygen atom to form CO2 and reform a molecule of NO2.
Elementary steps in a mechanism are almost always either unimolecular (involving one reactant molecule) or bimolecular (involving two reactant molecules). This is because the chances of having three distinct molecules collide and simultaneously interact is vanishingly small.
In contrast to the overall reaction, the rate law for elementary steps can be determined from their chemical equation as written. Unimolecular steps are first order. Bimolecular steps are 2nd order (first order with respect to each of the molecules). The reason this can't be done for overall reactions, is that for overall reactions we don't know just looking at the equation "how" the chemistry is happening. Elementary steps in a mechanism are a direct description of how the chemistry is happening. Therefore, we can write the rate law for each reaction in the mechanism simply from its balanced equation.