A job, or occupation, is a person's role in society. More specifically, a job is an activity, often regular and often performed in exchange for payment ("for a living"). Many people have multiple jobs (e.g., parent, homemaker, and employee). A person can begin a job by becoming an employee, volunteering, starting a business, or becoming a parent. The duration of a job may range from temporary (e.g., hourly odd jobs) to a lifetime (e.g., judges).
An activity that requires a person's mental or physical effort is work (as in "a day's work"). If a person is trained for a certain type of job, they may have a profession. Typically, a job would be a subset of someone's career. The two may differ in that one usually retires from their career, versus resignation or termination from a job.