Life exptectancy
Life expectancy is the average number of years of life remaining at a given age. It is the average expected lifespan of an individual. Life expectancy is heavily dependent on the criteria used to select the group. In countries with high infant mortality rates, the life expectancy at birth is highly sensitive to the rate of death in the first few years of life. In these cases, another measure such as life expectancy at age 5 (e5) can be used to exclude the effects of infant mortality to reveal the effects of causes of death other than early childhood causes.

Variation in the world today
There are great variations in life expectancy worldwide, mostly caused by differences in public health, medical care and diet from country to country. Climate may also have an effect, and the way data is collected may also be an important influence. According to the U.S. Census Bureau, Andorra has the world's longest life expectancy of 83.5 years.
There are also variations between groups within single countries. One example of this is the age expectancy between non-Latino Whites and African Americans in the U.S. Non-Latino Whites are expected to live until 78 and for African Americans its only 71. Significant differences still remain in life expectancy between men and women in France and other developed countries, with women outliving men by five years or more. To get some statistics, on average women tend to live until 80 years old whereas men are only expected to live until 74. These gender differences have been increasing in recent years. Poverty, in particular, has a very substantial effect on life expectancy. In the United Kingdom life expectancy in the wealthiest areas is on average ten years longer than the poorest areas and the gap appears to be increasing as life expectancy for the prosperous continues to increase while in more deprived communities there is little increase. However, in Glasgow the disparity is among the highest in the world with life expectancy for males in the heavily deprived Calton standing at fifty-four — twenty-eight years less than in the affluent area of Lenzie, which is only eight kilometres away. Life expectancy may also be reduced for people exposed to high levels of highway air pollution or industrial air pollution. Occupation may also have a major effect on life expectancy. Well-educated professionals working in offices have a high life expectancy, while coal miners (and in prior generations, asbestos cutters) do not. Other factors affecting an individual's life expectancy are genetic disorders, obesity, access to health care, diet, exercise, tobacco smoking, and excessive drug and alcohol use.
As pointed out above, AIDS has recently had a negative effect on life expectancy, especially in Sub-Saharan Africa.

Source: http://en.wikipedia.org/wiki/Life_expectancy