Step 1: Definition:
A class system is a form of social stratification in which people are ranked mainly on economic criteria such as wealth, income and occupation, rather than on birth.
Step 2: Key feature:
Unlike caste, class boundaries are relatively open: membership is achieved rather than purely ascribed, so individuals can move between class positions through education, occupation or economic success.
Final Answer:
Class system: stratification based on wealth/occupation, with open, achievement-based, and mobile membership, unlike birth-based systems such as caste.