A Public Cloud shares a pool of resources that are made available to everyone on the internet.
Security is controlled by user accounts with each user logging in to access their own resources within the common pool. Some public Clouds are free, for example Facebook or Gmail, and others are paid-for services such as Office 365 or Google Apps. This is the most common type of Cloud used by individuals and SMEs.
The main advantage of Public Clouds over the other types are flexibility and cost. The disadvantage is that Public Clouds are the least secure of the three types although the level of security is more than adequate for most purposes.
Private Clouds involve a secure Cloud based environment in which only the specified client can operate. Private Cloud services may be hosted internally, externally or both and are usually accessed across encrypted connections via public networks or by private leased lines.
Private Clouds are normally provided by specialist companies such as VMWare or Rackspace. With even the most basic single server packages costing several hundred pounds per month, Private Clouds tend to be aimed more at larger enterprises or businesses with very high security requirements.
Originally the term ‘Hybrid Cloud’ described a system in which an organisation used Private Clouds for some services and Public Clouds for others, but more recently it’s become used in a broader sense with various suppliers and manufacturers describing their offerings as Hybrid Clouds because they have elements of both Public and Private Clouds. Two examples of these are as follows:
Some network storage devices now come with a Cloud system that enables users to access them from across the internet. Because they are inside the company network, these devices are dedicated to the organisation that owns them (like a Private Cloud) but they are also accessible through the internet with a username and password (like a Public Cloud.)
Some Cloud providers are offering Virtual Servers which need to be accessed through secure channels (like a Private Cloud) but the underlying resources are shared between more than one organisation (like a Public Cloud.)