A substitution reaction is one with the general form;

For example;

For a nucleophilic substitution, Z is the nucleophile.

This nucleophile will attack an area of a molecule which has partial positive charge - or in other words is electrophilic. In the example above, the carbon-iodine bond is polar (because iodine is more electronegative than carbon), so there is partial positive charge on the carbon, and partial negative charge on the iodine; thus the carbon is attacked. Nucleophilic substitutions split into two main categories; 'S_N1' and 'S_N2' - this stands for;
Capital S - Substitution
Subscript N - Nucleophilic
...the numbers will be explained as the section proceeds.

S_N1

An example of the full mechanism of an S_N1 reaction is as follows;

Points to note:

i. OH^- is the nucleophile here, and I^- is called the Leaving Group (often abbreviated 'LG').

ii. The Rate Determining Step (RDS)  is unimolecular, hence the '1' in S_N1.

iii. The cationic intermediate is trigonal planar (this will be important when stereochemistry is considered).

Because the RDS is unimolecular, the reaction will display first order kinetics, where the rate depends only on the concentration of the starting material - in this case methyl iodide.

S_N2

An example of the full mechanism of an S_N2 reaction is as follows;

Points to note:

i. All of the reactants are the same as for the S_N1 reaction - this shows that both mechanisms can occur for the same overall reaction, and the delicate balance of competition between the two is controlled by many factors, as explained more fully below.

ii. The RDS is bimolecular, hence S_N2.

iii. There is no charged intermediate, but a trigonal bipyramidal transition state (abbrev. TS) with both the nucleophile and the leaving group partially attached to the p orbital created by the sp² hybridization of the TS.

Because the RDS is bimolecular, the reaction will display second order kinetics, where the rate depends on the concentrations of both the methyl iodide and the hydroxide.