The US does not have class systems.
It does however have unofficial classes - based on economic status. There's the upper class (rich people), middle class (somewhere in the middle), and lower class (poor people). There are certain characteristics that determine which category your fall in - for example, one's salary, $ in savings, where you live (type of house/neighborhood) etc. Most people fall into the middle class category, which is sometimes divided into two - upper middle class & lower middle class >> this depends on how close you are to either upper / lower class. Keep in mind that all of these classes are UNOFFICIAL. Nobody comes and gives you a definition, it's just a way of determining the average economic status of the nation and people.