Private clouds are not much different than public cloud offers, when you consider core patterns. Like their public cloud provider counterparts, private clouds provide virtualization to maximize the use of all connected systems, and some sort of tenant management subsystem to provide self-provisioning of resources. Private clouds also provide security and governance subsystems. The idea being that you can allocate resources as needed, such as databases, application servers, and storage systems, from the private cloud to support your business systems.The software that makes up private clouds is typically a virtualization system such as the very popular VMWare, or open source products such as Xen or Eucalyptus. However, there is no standard for what makes up a private cloud. There are even guys like 3Tera that have a public cloud offering, but also provide their software for use in building a private cloud as well.
While many private clouds are completely contained in the data center, many are virtual. Amazon, for instance, has a virtual private cloud version of their cloud computing offering which includes the best of both worlds. Virtual private clouds allow you to access the provider's cloud, in their data center, but you do so within infrastructure that you alone can access, and have control over.
Just a short time ago there was some confusion around the rise of private cloud computing, and many were pushing back on the concept. If you think about it, they had a point. The purpose of moving toward cloud computing is all about cheaper and more scalable resources, and the use of private clouds diminishes both benefits.
However, the use of a private cloud architecture is typically more effective and efficient than traditional one-server/one-application ways of filling up a data center. In many instances, organizations leverage private clouds because the CIO wants the architectural benefits of public cloud computing, such as cost efficiencies through virtualization, but is not ready to give up control of data and processes just yet. Indeed, I've found that to be the most common scenario.
I think we'll end up moving to a tiered approach. I think that many enterprises will stand up private clouds today, and then at some point learn to leverage public clouds, likely through dynamic use of public cloud resources to support bursts in processing on the private cloud. Many are calling this "cloud bursting," but it's a great way to leverage the elastic nature of public cloud computing without giving up complete control. Many private cloud systems, such as Eucalyptus, are interface-compatible with Amazon to support such an architecture.
Cut it any way you want, the use of the best of cloud computing within our own data centers is a small step in the right direction. Considering the dysfunctional state of most enterprise architectures out there, any progress is productive progress.Many organizations like the idea of cloud computing, but they don't like the idea of handing over their processes and data to a third party. Thus, the private cloud was born. It's providing a nice evolution for cloud computing, and is becoming a very effective architectural pattern.