The Sims 3 is built upon the same concept as its predecessors. Player controls their own Sims in activities and relationships in a similar manner to real life. The gameplay is open-ended and indefinite.

Sim houses and neighborhoods are entirely in one continuous map. The developers stated, 'What you do outside your home now matters as much as what you do within.' The game includes an optional feature called 'Story Progression', which allows all Sims in the neighborhood to autonomously continue as if the player were controlling them, such as grow up, get married, get jobs and promotions, have children, build or buy their dream house while the player isn't playing.

The Sims 2 used a reward system called Wants and Fears. This is replaced with a new system called Wishes in The Sims 3. Fulfilling a Sim's wish contributes to the Sim's Lifetime Happiness score and mood. Some wishes, such as 'Go to the Park', may add little points to their lifetime happiness while 'Have A Baby Boy' may add thousands of points. In The Sims 2, Wants and Fears also contributed to a Sim's 'Aspiration' meter, roughly analogous to current self-esteem. In The Sims 3, Aspiration is removed entirely, replaced with 'Moodlets', which contribute positive or negative values to the original Motivation meter. Moodlets can be inspired by physical events, such as having a good meal or comfort from sitting in a good chair, as well as emotional events like a first kiss or a break-up. Most moodlets last for a set duration, but some negative Moodlets can be cured (such as the one incurred by an urgent need to urinate) and some positive ones rely on the Sim's surroundings and traits.

Sims live for a set duration of time (adjustable by the player) and advance through several stages (baby, toddler, child, teen, young adult, adult, and elder). Sims can die of old age or they can die prematurely from causes such as fire, starvation, drowning, electrocution, (as of the World Adventures expansion pack) The Mummy's curse, (as of the Ambitions expansion pack) a meteor, and (as of the Late Night expansion pack) by thirst (vampires only).

Source: Wikipedia