What's the difference between Sn1 and Sn2 reactions?

The term Sn is drived from 'nucleophilic substitution reaction'. 1 means unimolecular, and the rate of the reaction only depend on the concentration of one reactant; 2 means bimolecular, and the rate of the reaction depends on both two reactants.

The tricky part is that for Sn1 there are two possible products whereas for Sn2 there only one possible product. Students easily get confused in this part, but if we look at the machenisms of those two substitution reactions, things will become more clearly (with the aid of the graph). There are a lot of factors that will influence whether a reaction will go through Sn1 or Sn2.

However, in A-level, we only need to consider the steric hindrance of the substrate, in other words, is the substrate bulky or not? If it is a bulky substrate, e.g.(CH₃)CBr, it will go through Sn1 route as it is hard for nucleophile to approach the centre carbon, and the only way is the Br leaving first. If it is not bulky, Sn2 will take place.

There are also other very interesting factors I can explain, if you want learn more chemistry.