Cloud computing is, in effect, a type of 'shared computing', comparable to grid computing, where a group of devices form together to deal with a particular process. Originally manipulated for high performance processing, cloud computing has become very much part of the public domain and cloud computing solutions are now sought after by individuals and businesses alike.
There are three main service models for Cloud solutions, IaaS, PaaS & SaaS;
IaaS, or Infrastructure as a Service, involves the use of physical or virtual devices. The cloud user is responsible for maintaining their cloud services, including the operating system and software.
PaaS, or Platform as a Service, is where cloud providers deliver a fully managed cloud solution, meaning that developers do not have additional expenditure such as through the purchase of software or hardware. Additionally, the provider will fully manage the solution.
SaaS, or Software as a Service, has slowly taken off over the past couple of years. A commonly known example of SaaS is the Google Apps platform - the client will only see a single access point to the cloud, however the cloud provider will be hosting their software across a number of virtual devices.