Sims can own cats, dogs, "womrats" (creatures similar to guinea pigs, whose name is a portmanteau of wombat and rat), and birds. Pets features 72 pre-made breeds of dogs and thirty pre-made breeds of cats. Pets can be acquired in five ways: over the phone, in "Create-A-Pet", in a pet shop, found as a stray dog/alley cat or by having a controlled Sim give or sell a pet to another Sim. Create-A-Pet is similar to "Create-a-Sim", where one builds human Sim characters. In Create-a-Pet, players may customize a cat or dog by choosing patterns and colors of coat, body shape, facial structure, and personality, which determines playfulness, neatness, and intelligence.
Pets have motives like their Sim counterparts. Sims and pets are similar except for the absence of the "Environment" motive, which "Scratching" replaces in cats and "Chewing" in dogs. Sims may also train their pets to do various tricks. However, a new sub-neighborhood is not added in the expansion pack. Instead, the expansion pack focuses almost entirely on pets.
Pets can follow one of three career paths: Security, Showbiz and Services. When pets get promoted, various pet-related "unlockables" are made available along with a code so players can share these items with others. These jobs (if pets do well) can be a great source of money.
Pets have three life stages: Puppy/Kitten (which lasts five days), Adult (which lasts 25 days for cats & small dogs; 21 days for large dogs), and Elder (which lasts 5-10 days). The length of a pet's elder hood is selected at random, however there is an aspiration reward object which will extend the pet's life by a few days with each use.
Pets are able to breed, and doing so will produce a litter. The maximum number of pets in a household is 6. A litter consists of 1-4 puppies or kittens. The number of puppies or kittens is dependent on the number of Sims and pets currently in the house. Players can play with up to eight Sims or six pets at one time, although one can only have a total of 10 Sims and pets in a household altogether.
Pets, unlike Sims, cannot be directly controlled without the use of a cheat code. A player can view the pet's action queue when it decides to take an autonomous action, but can not tell a pet to do something else. In addition, pet actions can not be canceled. Instead of direct control, a Sim must interact with a pet to increase the likelihood that it will perform or avoid various actions. It is up to a Sim to direct a pet's "learned behaviors" through either positive or negative reinforcement. For example, if a dog decides to relieve its bladder outside, a Sim can opt to praise the behavior in order to increase the likelihood that the pet will repeat that action. A pet's ability to learn certain behaviors is influenced by its overall personality. This also applies to tricks, such as rolling over, sitting up, coming when called, etc., which pets do not perform autonomously.