What does "going virtual" really mean in today's IT world? Virtualization as a concept is not new; computational environment virtualization has been around since the first mainframe systems. But recently, the term "virtualization" has become ubiquitous, representing any type of process obfuscation where a process is somehow removed from its physical operating environment. Because of this ambiguity, virtualization can almost be applied to any and all parts of an IT infrastructure. For example, mobile device emulators are a form of virtualization because the hardware platform normally required to run the mobile operating system has been emulated, removing the OS binding from the hardware it was written for. But this is just one example of one type of virtualization; there are many definitions of the term "virtualization" floating around in the current lexicon, and all (or at least most) of them are correct, which can be quite confusing. This paper focuses on virtualization as it pertains to the data center; but before considering any type of data center virtualization, it's important to define what technology or category of service you're trying to virtualize. Generally speaking, virtualization falls into three categories: Operating System, Storage, and Applications. But these categories are very broad and don't adequately delineate the key aspects of data center virtualization. It's helpful to distill these broader categories into eight, specific categories to thoroughly understand the differences (and similarities) between the definitions of virtualization.