In order to classify objects and terms, we need to be able to recognize both what an object or term is and what it is not.
One term that we can put through the process of classification is family. We know that families are groups of individuals, and that the individuals in the family are related though blood or marriage. Not all families are the same, obviously; some of the more common ways to classify families are through socio-economic levels, religion, and race. To further define socio-economic categories, we can say that there is an upper class, middle class, and lower class. Of course, within these classifications we can define further, such as upper middle class and lower middle class families. A more common vernacular for lower class families is to call them poor families.
The most obvious reason that families are called poor is the amount of money that a given family makes. There is a great deal more to being poor than not having as much money as rich people. To further define poor families, we must talk about what poor families experience. Perhaps the best phrase to describe the situation of a poor family is to say they live "paycheck to paycheck." This means that these families are unable to cover any additional expenses that come up outside of their regular income.