Developing countries generally referring to the countries of Africa, Asia, and Latin America is a term that was inspired by Walt Whitman Rostow’s classic work, The Stages of Economic Growth(1960). Rostow argued that all countries go through a series of stages of economic development from “underdeveloped” to “developed”; that the United States, Western Europe, and Japan had reached the “highest stage” or “developed-country” status; and that those countries that were not mature, developed capitalist countries were in the process of “developing” and moving through the required stages.

A variety of terms have been used to refer to these “developing” countries. These include less-developed countries, underdeveloped countries, undeveloped countries, backward countries, Third World countries, and newly industrializing countries. Except for Third World, which was advanced in the late 1960s and early 1970s to refer affirmatively to countries that were politically independent of the United States and the Soviet Union, these terms are more-or-less pejorative. Newly industrializing is more specific than the other terms, in that it refers to a limited number of countries that have begun industrializing since the 1970s.

Developing countries tend to have some characteristics in common. For example, with regards to health risks, they commonly have low levels of access to safe drinking water, sanitation and hygiene; energy poverty; high levels of pollution (e.g. air pollution, indoor air pollution, water pollution); a high proportion of people with tropical and infectious diseases (neglected tropical diseases); a high number of road traffic accidents; and generally poor infrastructure.
Often, there is also widespread poverty, low education levels, inadequate access to family planning services, corruption at all government levels, and a lack of so-called good governance. The effects of global warming (climate change) are expected to impact developing countries more than wealthier countries.

Development can be measured by economic or human factors. Developing countries are, in general, countries that have not achieved a significant degree of industrialization relative to their populations, and have, in most cases, a medium to the low standard of living.

Access to water, sanitation, and hygiene (WASH) services are at very low levels in many developing countries. In 2015 the World Health Organization (WHO) estimated that "1 in 3 people, or 2.4 billion, are still without sanitation facilities" while 663 million people still lack access to safe and clean drinking water. The estimate in 2017 states that 4.5 billion people currently do not have safely managed sanitation. The majority of these people live in developing countries.

Water pollution is a major problem in many developing countries. It requires ongoing evaluation and revision of water resource policy at all levels.
It has been suggested that water pollution is the leading worldwide cause of death and diseases and that it accounts for the deaths of more than 14,000 people daily. India and China are two countries with high levels of water pollution: An estimated 580 people in India die of water pollution-related illnesses.