There are two ways to define working class:
1. The most significant is how much money you make. One definition I've read is that a household is working class if its yearly gross income is below $30,000. Obviously income has a tremendous influence on how you live and the quality of your life. Working class people make so little money that they can't save much and have no security. Middle class people may, and often do, CHOOSE not to save money, but they could if they wanted to. Working class people have an income so low that they always have to worry about money, and cannot afford what most Americans consider a reasonable lifestyle. Working class people make less than a "living" wage, a wage on which you can support a family. They are being exploited by the middle and upper classes, who use working class people to do their shit work, to produce cheap goods and services so the middle and upper classes can keep their lifestyle.
2. A less significant but important definition is based on the kind of work you do. Working class people do work that is routine, boring and does not require advanced training or much thought. They are supervised by a middle class person and have very little control over their work, and therefore high stress. They have no job security and are frequently laid off. They are less likely than middle class workers to have health insurance. As they get older they tend to wear out sooner and find it increasingly difficult to stay employed, because employers don't want them. Working class people are expendable.
There is nothing mysterious about either definition, and people who don't get the distinction don't want to, because they benefit from middle class privilege and understanding class differences would make them uncomfortable.
Middle class people are the enemy.