Most of the pet pages have games to earn the virtual money you use at the pet page (also virtual) to buy things for your virtual pet. Most of the pet pages also have a virtual bank, virtual shopping areas, and at least one virtual battle arena. At some sites, you can also make your own virtual shop to sell things in.
You have to feed, groom and play with the pets you adopt, so they don't get sad or angry. To get food, toys and stuff to groom your pet with, you need money, and to get money you need to play games or sell things, or get it in a random event.
See also: digital pet