Every Sim has a personality, which you will choose when creating your Sim for the first time. Afterward, you can change your Personality for 50 SimCash any time, but this is expensive and may cost you real money. Your Sim can only have one personality, unlike Traits where you can have up to six .
Personalities when first introduced determined the more superficial things in the game, such as what TV stations you can watch; no one personality seemed to have an advantage over another. As of February 2012 however, the Sims Social is starting to add the idea of more specialized advantages to having one personality over another.