Unfree labour includes all forms of slavery, and related institutions (e.g. debt slavery, serfdom, corvée and labour camps). Many of these forms of work may be covered by the term forced labour, which is defined by the International Labour Organization (ILO) as all involuntary work or service exacted under the menace of a penalty.
If payment occurs, it may be in one or more of the following forms:
Unfree labour is often more easily instituted and enforced on migrant workers, who have traveled far from their homelands and who are easily identified because of their physical, ethnic, linguistic, or cultural differences from the general population, since they are unable or unlikely to report their conditions to the authorities.
According to the Marxian economics, under capitalism, workers never keep all of the wealth they create, as some of it goes to the profit of capitalists. By contrast with modern subjective theory of value (as used by neoclassical economists), the wages offered necessarily represent the marginal utility of the labour, and any profit (or loss) is also due to other inputs provided, such as capital, time value of money, or risk.