Forced labor is defined by the ILO (International Labour Office) Forced Labour Convention, 1930 (No. 29) as "all work or service which is exacted from any person under the menace of any penalty and for which the said person has not offered himself voluntarily".
There is an obvious overlap between forced labor situations and slavery-like practices. A person in a situation of slavery will certainly be forced to work, but this is not the only defining feature of the relationship.
For purposes of international law, ILO focuses on specific practices identifying a situation of forced labor, namely practices that typify the lack of consent of the forced laborer, and those that define the threat to which the latter may be exposed in case of non-compliance. To quote the text of the convention:
Lack of consent to work
Birth/descent into "slave" or bonded status;
Physical abduction or kidnapping;
Sale of person into the ownership of another;
Physical confinement in the work location - in prison or in private detention;
Psychological compulsion, i.e. an order to work, backed up by a credible threat of a penalty for non-compliance;
Induced indebtedness (by falsification of accounts, inflated prices, reduced value of goods or services produced, excessive interest charges, etc.);
Deception or false promises about types and terms of work;
Withholding and non-payment of wages;
Retention of identity documents or other valuable personal possessions;
Menace of a penalty - Actual presence or credible threat of:
Physical violence against worker or family or close associates;
Threat of supernatural retaliation;
Imprisonment or other physical confinement;
Denunciation to authorities (police, immigration, etc.) and deportation;
Dismissal from current employment;
Exclusion from future employment;
Exclusion from community and social life;
Removal of rights or privileges;
Deprivation of food, shelter or other necessities;
Shift to even worse working conditions;
Loss of social status.
For statistical purposes, in order to assess the magnitude of numbers of people victims of "forced labor practices", ILO established categories as follows:
Forced labor imposed by the State including
Forced labor exacted by the military;
Compulsory participation in public works;
Forced prison labor (forced labor camps but also work imposed in modern semi privatized or fully privatized prisons; forced labor imposed by rebel groups is also included in this category).
Forced labor imposed by private agents for commercial sexual exploitation including
Women and men who have involuntarily entered prostitution or other forms of commercial sexual activities, or who have entered prostitution voluntarily but who cannot leave. It also includes all children who are forced into commercial sexual activities.
Forced labor imposed by private agents for economic exploitation comprising
all forced labor imposed by private agents other than for commercial sexual exploitation. It includes, among other things, bonded labor, forced domestic work, or forced labor in agriculture and remote rural areas.
In addition to this broad typology, a distinction is drawn between forced laborers who were trafficked and those who were not trafficked.
Human trafficking affects millions of men, women and children every year, and is explicitly and precisely defined by the Palermo Protocol, which supplements the United Nations Convention against Transnational Organized Crime.
"Trafficking in persons" means the recruitment, transportation or receipt of persons, by means of the threat or use of force, of abduction, of fraud or deception, for the purpose of exploitation including the prostitution or sexual exploitation, forced labor, slavery, or the removal of organs.
The consent of the victim of trafficking is irrelevant where any of the mentioned coercion means have been used. If children (anyone under 18) are concerned, the process will be considered "trafficking in persons" even if it does not involve any of the listed coercion means.
[Adapted or quoted from: ILO Forced Labour Convention (No. 29); ILO Abolition of Forced Labour Convention (No. 105); Protocol to Prevent, Suppress and Punish Trafficking in Persons, Especially Women and Children, General Assembly resolution 55/25, annex II, p.32; and Global Report under the Follow-up to the ILO Declaration on Fundamental Principles and rights at Work 2005]