Rate Determining Step
When a reaction is the result of a series of elementary processes, the rate of the overall reaction is determined by the slowest reaction in the sequence.
eg. Ag+(aq) + Cl-(aq) ---> AgCl(s) 
a simple reaction involving only a single 1 step mechanism therefore very fast.
eg. 2 NO(g) + 2 H2(g) ----> N2(g) + 2 H2O(g) 
This appears to be a 4 particle reaction as written. Impossible!!
The reaction mechanism for the above reaction has been determined to be:
step #1 2 NO + H2 ----> N2 + H2O2 slow since it invovles a rare three particle collision. This is the slowest and therefore is the step that determines the rate of all the other steps.
step #2 H2 + H2O2 ------> 2 H2O fast reaction