Petz is a virtual pet series, originally created by P.F.Magic (but changed hands a couple of times later) centred around raising virtual cats and dogs. There were five major installments in all, starting with a simple version released in 1995 where you only adopted one pet, to allowing multiple pets to interact and breed. "Catz" and "Dogz" versions were sold seperately but could both be installed on the same system to allow the two species to play together. The final installment, Dogz5/Catz5, was released in 2002.
The games spawned a whole virtual community, some of which is still around today even though the games have been out of production for over a decade. Sites were built devoted to breeding, showing, and games and awards. Probably one of the main reasons for the games' staying power was that breed files could be modded. The pet were made up of a structure consisting of "ballz" joined together with "linez" that could be edited to create new shapes. The process was dubbed "hexing" as a lot of early work was done using a hex editor. It's not a very accurate term, but it stuck.
For a lot of people online today, Petz is the go-to game for a good nostalgia moment. Best of all, you can still play it, and so do plenty of other people! To run Petz 4 on a modern Windows system, follow the instructions over at Petz 'R Us. I can confirm these work for Windows 7 and 10. (For other systems I hear it can be made to run using Wine, but have no experience myself.)
Note: after Petz 5, the games were reworked into a cutesey 3D series that bears little or no relation to the originals, but is what you'll probably find if you go looking for "petz" today. These games are related to the original in name only and this page has nothing to do with them.