Personality

A Sim's personality is determined by their Zodiac Signs. In both games, personality was split into 5 sections: Niceness, cleanliness, outgoingness, activeness and playfulness. Whilst in The Sims, this did little to the nature of the Sim, in The Sims 2 it makes quite an impact. In The Sims 3 personality was replaced with traits.


 * Outgoing Sims will often use a mirror autonomously. The social bar of outgoing Sims decay rapidly, as they desire social interaction. Due to their social nature, outgoing Sims gain Charisma faster.
 * Shy Sims dislike social activities, and if told to do so, will often grimace at the camera, and walk to the activity slowly with head bowed. Their social bar decays more slowly.


 * Nice Sims are usually kinder to other sims, and will probably respond positively to social interactions, even if they aren't interested in it.
 * Mean Sims are more likely to cheat in games and react negatively to social interactions
 * Playful Sims can jump on sofas and play in bathtubs. They will also be able to skip to get somewhere.


 * Serious Sims dislike watching TV and prefer reading. Should such a Sim be told to read they will smile at the camera and punch the air. However, though serious Sims autonomously choose reading and the like over TV, the actual gain to Fun from watching the TV is no different than if a Playful sim watched it. Serious Sims also gain Logic faster, while Playful sims gain Creativity faster.


 * Clean Sims like cleaning activities, even gaining fun off them. They may autonomously 'groom self' in the mirror. Because of their clean attitude, their hygiene decays slower. Clean sims gain the cleaning skill quicker as well.


 * Sloppy Sims prefer to make messes and can also rummage through the trash can for simoleons. They dislike cleaning, and actually lose a small bit of fun when forced to do it. Due to their messy nature, their hygiene decays faster, though they seem to care less about it. Also, they tend to flatulate more often than other Sims.


 * Active Sims lose energy slower than anyone, and they enjoy working out. They will also be able to run to get somewhere. However, due to all this activity, their hunger decays faster too. Active Sims also gain Body skill faster than normal, due to their active nature.


 * Lazy Sims tend to sleep more than other sims, since their energy bar decays faster. Also, due to their 'couch potato' nature, their comfort decays faster too, making them need to sit or relax more often. And, as a side effect to all of this laziness, their hunger decays slower.

Pet personality
In The Sims 2: Pets, a pet's personality is split into 5 categories: Intelligence (Doofus to Genius), Activeness (Hyper to Lazy), Friendliness (Independent to Friendly), Aggression (Aggressive to Cowardly), and Cleanliness (Pigpen to Finicky).

Each category has three values, the ones at each end, and an in-the-middle average value. For example, a pet could be Hyper, or Lazy, or somewhere in between, but pets don't get the zero-to-ten scale that human Sims do.

Sims 3
In the Sims 3 you may select up to 5 personality traits that help diversify the kinds of people you will meet. A list of them may be found here