In Marxist economic theory, surplus value is the difference between the value of the goods or services produced by a worker's labor power and the wages the employer pays for that labor power; it is the value of the worker's labor that is 'left over' after the employer has covered all the costs involved in hiring the worker, including wages and the costs of production. According to Marx, this surplus value is the source of capitalist profit and represents the exploitation of the proletariat (working class) by the bourgeoisie (capitalist class) because the workers do not receive the full value of what they produce; instead, the surplus is appropriated by the capitalists who own the means of production. The extraction of surplus value is seen by Marx as a fundamental characteristic of capitalism and a key driver of the inherent conflict between the interests of capitalists and workers.