A Sim's interests are randomly generated when created or at birth. The more points in an interest, the more interested a Sim is in the particular subject. The interests that have the highest points are the ones a Sim will usually talk about when they initiate a conversation or be interested in when being talked to. For the most lengthy and constructive conversations, it's best to talk to Sims that have similar interests.
In the original release of The Sims, there were 8 interests; Travel, Money, Politics, 60's, Weather, Sports, Music, and Outdoors. For children, the first four were replaced by Toys, Aliens, Pets, and School. The Hot Date expansion pack added 7 more interests that were only available to adults; Exercise, Food, Parties, Style, Hollywood, Technology, and Romance. Hot Date also introduced magazines, which adult Sims can read to change their interests.
Interests run on a 10-point scale, from 0 to 10.00. If a Sim has between 0 and 3.0 points in an interest, he or she will react negatively to it. If a Sim has between 3.01 and 6.0 points, he or she will react somewhat positively, though the thought bubble for the interest will have a red X through it. If the Sim has at least 6.01 points in an interest, he or she will react positively, and the interest will appear when the player uses the Change Topic interaction.
Only adults can use the Change Topic interaction, and they can only use it when they're involved in a group activity. It's not available when two Sims are just talking to one another, whether face-to-face or on the phone.
The Sims 2
There are 18 interests, and they're the same for all ages. The interests are Paranormal, Sci-Fi, Health, Politics, Culture, Environment, Food, Travel, Weather, Crime, Money, Work, Entertainment, Fashion, Sports, Animals, School, and Toys. A controllable Sim's interests can be found on the Simology panel.
Interests run on a 10-point scale. In general, Sims with at least four points in an interest will react positively to it. Kids can use the Change Topic interaction, and it is available when Sims are just chatting with one another, as well as in group activities. When using the Change Topic interaction, the player will be able to choose any interest, regardless of how many points the Sim has in it.
Interests can be changed by reading magazines or through the "Share Interests" Social Interaction.
Interests are unlike personality points when you increase them. It is impossible to have 10 in every single interest because once you increase an interest by one point another one will decrease by 1 point.