The proletariat (often shortened to "prole" in slang) is a term used by communists for a segment of society. It describes the class of workers in urban areas who work in industry or manufacturing. These urban workers are different from the peasants, who are workers who do farming jobs in rural areas. Someone is who part of the proletariat class is called a proletarian.
The political way of thinking of communism states that the proletariat should have a revolution and take over the means of production. They are often sarcastically referred to as "wage-slaves."
The proletariat is the laboring bulk of society that does most of the useful, necessary work, and pays most of the taxes. It is also perceived by capitalists as being necessary for those reasons: labor and taxes, plus being available to exploit to produce profits.