Cattle and Pets are ways to classify how you think about a compute unit that is involved in providing a service. Typically the unit would be a server ( metal or virtual ) but can also be a container ( e.g Docker ) that is running on a server. The compute units classification will determine how you treat it.
Can be destroyed and replaced at any time without the service failing. It’s Cattle
Indispensable; destroying and replacing will cause the overall service to fail. It’s a Pet.
EXCHANGE01 is ACME Corporations single server providing email to the entire company. If it goes down, email service is lost. EXCHANGE01 is a Pet
The cluster compromising virtual servers WWW001 through to WWW100 , provides ACME Corporation website to the world. Any server in that cluster can be destroyed and replaced without ACME Corporations website failing. Therefore these servers are Cattle