Self-Employment has existed since The Sims, when Sims could sculpt and paint using skill objects, or (with Unleashed) plant garden vegetables and sell their paintings, sculptures, and produce for money. Self-employment for a Sim, means using the Sim's skills and abilities to earn money. A Sim need not join a career track to earn money, though The Sims 3: Ambitions made it possible to register as self-employed, joining a skill career in the process, and earn even more money and track progress in the field.
The Sims 2Edit
In The Sims 2, Sims are able to paint, sculpt, garden, fish, and play music for tips. The Sims 2: Open for Business expanded this into retail careers, hair-styling, toy-making, robotics, and flower-arranging. Progress in Open for Business self-employment skills is tracked by Talent badges.
The Sims 3Edit
All of the skills used in skill careers can also be used to earn money without registering officially as self-employed and joining a skill career, but joining a skill career has the advantage of Simoleon bonuses and tracking progress.
The Sims 4Edit
The Sims 4 does not have skill careers, but Sims can still earn money through painting, woodworking, fishing, playing music for tips, hacking, and gardening. The Sims 4: Get to Work also adds a retail self-employment option, which includes selling baked goods. The Sims 4: Dine Out adds the option for Sims to run their own restaurant and The Sims 4: Cats & Dogs allows Sims to run vet clinics.