The term is commonly used more broadly to refer to cases of widespread malnourishment or deprivation among populations, usually due to poverty or adverse agricultural conditions; see famine and malnutrition for a discussion of this.
Hunger as a condition
The term hungry is commonly used simply to mean being ready for a meal. After a long time without food, the mild sensation of hunger associated with being ready for a meal becomes a progressively more severe sensation, until it becomes acutely painful. Prolonged hunger will drive people to eat substances with no nutritional value, such as grass and soil, simply to fill their stomachs. Eventually, after long enough without food, death will occur through starvation.
In contrast, fasting is the practice of voluntarily not eating for a short period of time.
Politics of hunger
As of 2004, hunger continues to be a worldwide problem. According to the Food and Agriculture Organization of the United Nations, "842 million people worldwide were undernourished in 1999 to 2001, the most recent years for which figures are available" and the number of hungry people has recently been increasing. 
There is a wide range of opinions as to why this problem is so persistent. Organizations such as Food First raise the issue of food sovereignty and claim that every country on earth (with the possible minor exceptions of some city-states) has sufficient agricultural capacity to feed its own people, but that the "free trade" economic order associated with such insititions as the International Monetary Fund (IMF) and the World Bank prevent this from happening. At the other end of the spectrum, the World Bank itself claims to be part of the solution to hunger, claiming that the best way for countries to succeed in breaking the cycle of poverty and hunger is to build export-led economies that will give them the financial means to buy foodstuffs on the world market.
Amartya Sen won his Nobel Prize in part for his work in demonstrating that hunger in modern times was not the product of a lack of food but rather (most often) in food distribution networks / governmental organizations in the developing world.