What is the difference between life span and how long you live?

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Very generally, life expectancy is the approximate amount of time an organism ought to live, taking into account average life spans for that organism in a specific environment, including all environmental factors, such as pollution or lack of pollution, health indicators and risk factors such as (in humans) smoking, obesity, cholesterol levels, job related factors and so on and so forth.

Life span is how long the organism actually lives. If, for example, we assert that an average American male non-smoker non-drinker who gets a reasonable amount of healthy exercise and does not engage in a whole host of possible risky behaviors is thought to have a life expectancy of, say, 78 years, that doesn't mean that individual will live to be 78. It means that the average life expectancy of other males in the same group is 78 years. Some will live longer. Some will live to 101. Some will be hit by a bus. Some will succumb to cancer regardless of lifestyle (no one yet knows exactly what causes all cancers). Life span can be considerably longer or shorter than life expectancy, depending on a whole host of factors that cannot all be controlled.