Sims with the Knowledge Aspiration want to know everything there is to know about anything. Make sure they have the time to study and explore every nook and cranny. Their lives are a simple equation: the more Knowledge, the higher the Aspiration Meter, and the longer and brainier the life.
Knowledge is an Aspiration in The Sims 2. Knowledge Sims are full of curiosity and always want to learn more and more. This frequently shows up in wants to learn skills, but they also want to learn things that allow them to do things such as discover secret vacation lots[TS2:BV].
Knowledge Sims' curiosity makes them relatively fearless. While other Sims fear things such as seeing ghosts, becoming a creature, and being abducted by aliens, Knowledge Sims often want these experiences. They may even want to get struck by lightning[TS2:S] They also like to encounter near-death experiences, such as defeating the Grim Reaper or being saved from death. Teenage Sims are eager to have good grades at school, and young adult Sims always want to be serious in their studies at college[TS2:U].
In the Windows and Mac OS X versions, the Sim will fear losing their creature status. Mac OS X has the same Wants and Fears as Windows with the exception of Wants and Fears introduced in FreeTime and Apartment Life since those 2 EPs were only released for Windows.
Knowledge Sims make good love matches for Family Sims, as they will usually want to get married and have at least one child, most importantly they will usually roll wants of teaching their children their toddler skills, teaching them how to study, or even getting them into Private School.
Knowledge Sims often make good companions for Fortune Sims, as they are both ambitious and their wants don't clash very easily. However they will only have a slight chemistry boost.
Knowledge Sims tend to get on pretty badly with Romance or Pleasure Sims, as they don't have much in common.
Despite not having much in common, Knowledge Sims often can have fairly good chemistry with Romance Sims. Even without FreeTime, a Knowledge Sim can attract a Romance Sim and vice versa. They can often have double-bolt attraction and get along fairly well if Nightlife is installed and they are able to date.
Knowledge and Romance aspirations can be an interesting combination, and if one partner is Romance/Knowledge with the other Knowledge/Romance, they will get along very well.
Knowledge Sims are also the only Sims who do not fear becoming a special being (Vampire, Werewolf, etc.). In fact, it can be used to boost their aspiration, curing it when it is no longer a fear to be cured. The want to become a special being is probably due to the fact that they want to know more about them, and what better way to learn about them than by being one?