A Sim's aspiration is their biggest goal. You can change your Sim's aspiration whenever you want. The first aspiration you choose when you are creating your Sim will give you a special trait, but changing your Sim's aspiration after that won't change your Sim's traits. Each aspiration gives your Sims a set of tasks to complete. When you complete all these tasks you will get a second special trait. The trait you get will be different for each aspiration. These can be completed if wanted using cheat codes in the game.