In nucleophilic substitution reactions (SN2), one group, called the leaving group, is exchanged for another, called the nucleophile. 
[1] Substrate - Species containing the leaving group
[2] Nucleophile - Lewis base capable of attacking atoms with positive polarity (or charge)
[3] Reaction product
[4] Leaving group - Species that leaves the substrate, being replaced by the nucleophile
Nucleophilic substitution reactions allow many organic compounds to be obtained from alkyne halides. Let's see some examples:
a) The reaction of bromoethane with sodium hydroxide produces ethanol. 
b) The reaction of isopropyl chloride with sodium cyanide produces a nitrile 
c) The reaction of iodomethane with sodium methoxide produces an ether (dimethyl ether) 
d) Amines can be obtained by reacting ammonia with haloalkanes 
In these examples the leaving group is represented in red and the nucleophile in blue.
