Many persons throw around the word "proletariat" as a fancy-sounding big word to mean "working class" or anyone who is "exploited for their labour" - but this is problematic. The proletariat is the working class that emerged in metropolitan areas after the Industrial Revolution when Capitalism was born.
The Industrial Revolution involved rapid urbanisation, and a great amount of division of labour. Whereas the peasantry could claim credit for their work, the jobs of the proletariat were split up into many mechanical, menial, and repetitive tasks. Each individual worker was therefore seen as disposable and replaceable, but they were collectively the blood of the factory. When organised, the proletariat is feared by the classes that exploit it.
The proletariat was not the first or only class to be exploited for its labour. Rural farm workers and enslaved people were not considered to be a part of "the proletariat" despite spending most of their time doing labour.
In industrialised countries, the proletariat is the class that is expected to overthrow Capitalism and establish Socialism. However, less developed countries do not experience Capitalism in the same way, and "the proletariat" may be a small portion of the population. In less developed countries, the proletariat would have to form an alliance with other classes to establish Socialism.