User blog comment:BlankyXP/Free Shipping On All Orders Over $75/@comment-4802717-20120911210252/@comment-1828812-20120913160937

@Potter: A moodlet is a Sims' mood reflection on events that happen to them.

Like say a sim eat something bad (spoiled food), their moodlet would come up as "Nausea" something like that. It's not limited to just that but they have moodlets based on social situtions as well like " New Friend" if the Sim made a new friend.