You take care of virtural cats or dogs and you can even breed them and have kittens or puppies. It takes forever for them to go up and fall in love, but there's a program you can download called "Petza" that lets you cheat by making the animals age instantaly and you can also breed them instantly. :wtg
The petz games were originaly created for people who couldn't have real pets (allergies, appartment living, etc.) or didn't want the responsiblites that come with owning living animals. You can pet you pet by holding down the mouse button when the cursor is on the animal (I wish BF could do something like this for ZT2.) You can also play with your pets with the toys that are included in the game (balls, frisbees, treats) and feed them. It's not a very popular game as far as I know because I only see it being sold in the stores around Christmas time.