To use the formal definition: A private cloud pools and dynamically allocates your IT resources across business units, so that services can be deployed quickly and scaled out to meet business needs whenever they occur. Usage of these resources can be tracked and billed back to each business unit. With private cloud you get many of the benefits of (public) cloud computing with the additional control and customization associated with using resources that are dedicated to your organization.
What's that all mean? It means that a private cloud takes the concepts of a dynamic datacenter to the next level. In a dynamic datacenter, we use virtualization to - for all intents and purposes - divorce hardware considerations from your IT workloads. The infrastructure you have siloed to different departments, buildings, campuses or what have you, can now be combined into one virtualized pool of resources - infrastructure that IT can offer as a service, quickly and elastically, anywhere in the organization where it's needed. Hence the moniker, Infrastructure as a Service (IaaS). Servers, platforms and applications run on virtualized servers that are quickly deployed and scaled without requiring much integration with the hardware layer. IaaS is currently the beating heart of a private cloud design, but Platform as a Service (PaaS) is coming soon to a private cloud near you (see below).
The private cloud enables this next-level of IT service, using identity management and advanced systems management tools to enable IT pros and even end users to build up, maintain and tear down resources that before would have required lengthy IT intervention. Take the case of a developer looking to test a new software product. Previously, she'd have to ring up IT and request a server be built to her testing specifications. Wait two weeks for IT to approve the request and someone might then get around to giving her a machine. Meanwhile, her testing process is in limbo. In a private cloud, she'll be able to log into a self-service portal, build her own virtual server decked out just the way she needs it, test till her head turns blue and then tear the whole thing down in the end. To the IT manager, this whole transaction will simply take place in his event and audit logs.
Does this mean he's out of a job? Heck, no. For one, the elements that comprise a private cloud are the same ones you need him for today - Windows Server 2008 R2, Active Directory, Hyper-V, System Center and more. For another, even with these platforms optimized into a working private cloud, you'll need to align these new capabilities with your company's workflows and business requirements. Yes, the IT pro role will likely need to evolve in this scenario. Grow from being solely a technologist to being able to strategize with technology - add new value to the business by combining technology expertise with business expertise . Find new ways of doing things and push that competitive edge.