The informal sector covers a wide range of labor market activities that combine two groups of different nature. On the one hand, the informal sector is formed by the coping behavior of individuals and families in economic environment where earning opportunities are scarce. On the other hand, the informal sector is a product of rational behavior of entrepreneurs that desire to escape state regulations.

The two types of informal sector activities can be described as follows:

1. Coping strategies (survival activities): casual jobs, temporary jobs, unpaid jobs, subsistence agriculture, multiple job holding;
2. Unofficial earning strategies (illegality in business): (1) Unofficial business activities: tax evasion, avoidance of labor regulation and other government or institutional regulations, no registration of the company; and (2) Underground activities: crime, corruption - activities not registered by statistical offices.

The concept of the informal sector was introduced into international usage in 1972 by the International Labor Organization (ILO) in its Kenya Mission Report, which defined informality as a “way of doing things characterized by (a) ease of entry; (b) reliance on indigenous resources; (c) family ownership; (d) small scale operations; (e) labor intensive and adaptive technology; (e) skills acquired outside of the formal sector; (g) unregulated and competitive markets”. Since that time, many definitions were introduced by different authors and the ILO itself. The ILO/ICFTU international symposium on the informal sector in 1999 proposed that the informal sector workforce can be categorized into three broad groups: (a) owner-employers of micro enterprises, which employ a few paid workers, with or without apprentices; (b) own-account workers, who own and operate one-person business, who work alone or with the help of unpaid workers, generally family members and apprentices; and (c) dependent workers, paid or unpaid, including wage workers in micro enterprises, unpaid family workers, apprentices, contract labor, homeworkers and paid domestic workers.

Operationalizing the concept of informality for the purpose of measurement is not easy both because the two categories of the informal sector overlap and because the border between the informal and the formal sector is blurry. First, if unofficial earning strategies are exercised by a low-profit small enterprise with low quality working conditions, then workers of this enterprise and the enterprise itself can be classified as belonging to both informal market categories. An example of such a case is an unregistered one-person low-profit street trade enterprise - these characteristics combine unofficial and survival activities. Second, some formal market jobs or enterprises can be classified as informal if it is found that they have poor work protection or if the life style and opportunities they entail are considered undesirable. If the street trader from the previous example registers her enterprise, the enterprise and the trader herself could be categorized as belonging to the formal sector if the profit is considered above the survival level.