Answer:
The proletariat is the unpropertied working class. They can be distinguished from small farmers or artisans in that they do not own any part of the means of producing anything. A farmer who owns his land and equipment owns some of the means he needs to produce agricultural produce; a cobbler or other small producer owns tools and other means to produce shoes or whatever else he makes. On the other hand, factory workers, miners and other proletarians had nothing to offer but their labour which they exchanged for wages.
Marx noticed that the industrial revolution had vastly increased this class of worker. Their position was similar to that of serfs or slaves in other economies, in that they sold their labour for the means of subsistence.
Marx also noticed that the industrial proletariat was being paid far less than the intrinsic worth of their labour, which could be measured by the value of the end product of the production process.