Earlier I had mentioned about Pets vs Cattle, and one of my recent friends asked me what it actually means.
Pets vs Cattle is a concept. Recently brought on by the advent of cloud computing and managing massive infrastructures which can be easily automated. Its extremely different than traditional systems administration, which was born from physical machines where you may need to purchase new hardware, build it, then deploy it within your datacenter.
First you give them a name. Then you installed services on that server, and if the server got corrupted or hardware failed you had to repair the computer, possibly rebuild the hard drive, fix corrupted filesystems, I could go on essentially you nursed it back to health. This is referred to as "taking care of a pet", you take care of it, you update, upgrade, and patch it. Some applications still need this attitude, but many in this decade do not! Now we've changed, and we with RESTful APIs can quickly spin up infrastructure, and build from scratch to a working machine in minutes.
Now with cloud computing we are able to do amazing things. I can easily build a "golden image" from a running application, even build that image from scratch and use it in an autoscaled environment (something Netflix has done for years!). Using tools like Ansible, Puppet, Salt, and Chef, you can quickly build new infrastructure that is known to work. You can even use those tools to build a brand new image which can then be used to create 100 servers just with one command line entry. Instead of names these guys get numbers. All servers are essentially identical to each other. If a server dies, you issue a couple API calls (or not if you are using AWS Autoscaling or similar, as it does it for you if you want it to), and now you have replaced that server in your environment. If a cow is ill/dying you kill it, and get another. That's where this analogy originates.