Sociology is the study of society: how people live together in groups, how they shape one another, and how societies stay stable or change over time.
Key concepts
Sociologists use a shared vocabulary: society (a group sharing a way of life in a place), culture (shared beliefs, customs and behaviour), norms (unwritten rules about behaviour, like queuing politely), values (beliefs about what is important, like honesty), roles (expected behaviour attached to a position, like being a pupil), and status (a person's position in society — ascribed if given at birth, achieved if earned).
Sociology versus common sense
Common sense is what "everybody knows" — but common-sense ideas are often based on personal opinion, and different people hold opposite beliefs. Sociology is different because it gathers evidence and tests ideas carefully before drawing conclusions. Common sense might say "people who commit crime are simply born bad"; a sociologist would collect data and might find that poverty, opportunity and how communities react all play a part.
How societies pass on their culture
Children are not born knowing the norms and values of their society — they learn them through socialisation.
A descriptive subject
Importantly, sociology describes and explains the different ways people live and the different views they hold. It does not tell you what to believe or which way of life is correct.