Lots of great info in the following article from IGN. Pets can have careers – police dog, ghost hunter; they have moodlets, challenges, traits…and best of all – over 100 breeds!
The Sims 3: Pets marks the first time that pets are on equal footing with your Sims, with moodlets, motivations, challenges, and personality traits. including options like piggy or hyper or non-destructive. When you create new Sims in the create-a-Sim menu, cats and dogs are now options for family members, with more than 100 breeds at your disposal (including my beloved breed the Keeshond — miss you, Casey!). And if you’re feeling mean to the humans in your family, allergic to fur is a new trait you can bestow.