Public cloud computing, such as that offered by Amazon, Microsoft, RackSpace and others, is the most prevalent deployment model in use today. The costs to enter are the lowest of the three models, and because it is heavily commoditized, it offers effective self-service options. This minimizes the need for on-site technology staff.
Private cloud computing, where the organization creates its own cloud computing infrastructure internally, or has it hosted/managed by a large scale data center provider, is an option that larger enterprises make use of. The costs are not trivial, and it can take a fair amount of time to put into place. It offers the most security and privacy and customization of the three models.