Each expansion pack for a The Sims is centered around one or more themes.
Family Life is the main theme of The Sims: Livin' Large and The Sims 3: Generations. While Livin' Large doesn't primarily focus on the interaction between families, it introduced several new features at a time when the series was new. Meanwhile, the main focus of Generations is on the family, where family time is made more elaborate, and Sims are meant to "Live the Life to the Fullest". The Sims 2: Family Fun Stuff adds several items related to families and children into The Sims 2.
Parties are the main focus of The Sims: House Party, in which the ability to throw a party was introduced for the first time. Parties are also enhanced in The Sims 3: Generations, where there are even more types of parties. The Sims 2: Celebration! Stuff provides more party objects for The Sims 2.
Dating feature is the main theme of The Sims: Hot Date, as well as one of the themes that come in The Sims 2: Nightlife, and again, The Sims 3: Generations. Dating feature enables two paired Sims to spend their time together doing romantic activities.
The Vacation feature is introduced in The Sims: Vacation, and is the main theme in The Sims 2: Bon Voyage, The Sims 3: World Adventures, and The Sims 3: Island Paradise. The main purpose of this theme is to show Sims other parts of the world in their respective games, and go give them ways to spend their time and relax while on vacation. In The Sims 3, World Adventures doesn't primarily focus on relaxation or vacations, but rather on exploration and adventuring. However, Island Paradise will.
Pets is a theme used in many games throughout The Sims series. Sims can keep pets as part of their life in The Sims: Unleashed, The Sims 2: Pets, and The Sims 3: Pets. Pets are also featured in many console games; as seen in here.
Fame is the main theme in The Sims: Superstar and The Sims 3: Showtime, as well as one of the themes in The Sims 3: Late Night. Sims are able to work in the Fame career, become celebrities, or become performers to pursue their desire to become famous. The Sims 3: Movie Stuff will add various themed items and outfits from popular Hollywood film genres.
The college theme is only featured in The Sims 2: University and The Sims 3: University Life, where Sims can seek and pursue further education at college. While at college, Sims will also encounter and experience the college life: throwing parties, joining a Greek house, or studying.
Nightlife theme is where the game mainly focuses the life at the big town in the clubs, and have outings. There's also the night creature that will make the nightlife more intriguing, vampires. This theme serves as the theme in The Sims 2: Nightlife and The Sims 3: Late Night. [clarification needed]
Sims can run businesses, in addition to going to work in careers, or perform job in their profession. These features enable Sims to earn money in ways other than just going to work with adequate skills. Sims can now perform the real work in their profession and business. This theme is mainly in The Sims 2: Open for Business and The Sims 3: Ambitions.
Seasonal activities and the seasons as well are introduced in The Sims 2: Seasons, where precipitation, such as rain and snow, will occur. Holiday themes also appears in The Sims 2: Holiday Party Pack and The Sims 2: Happy Holiday Stuff. A weather theme for The Sims 3 was introduced in The Sims 3: Seasons.
With the addition of reputation and social class, Sims don't just interact with other people and do things unnoticed. What Sims do towards others remains noticed as part of the Social Life theme in The Sims 2: Apartment Life. Social groups, a similar feature, is returning in The Sims 3: University Life.
The enhancement in bathroom design appears in The Sims 2: Kitchen and Bath Interior Design Stuff and The Sims 3: Master Suite Stuff.
Other stuff packs are exclusive to a theme only, which is categorized here.