Cloud computing is the delivery of computing as a service rather than a product, whereby shared resources, software, and information are provided to computers and other devices as a utility over a network (typically the internet). Cloud computing seems to have become an umbrella term under which these and other existing solutions can be remarketed. Cloud computing provides computation, software applications, data access, data management and storage resources without requiring cloud users to know the location and other details of the computing infrastructure.
Here are some types of cloud computing services:
SaaS (Software as a Service) delivers a single application through the browser to plethora of customers utilizing a multitenant architecture. It does not force any upfront investment in buying servers or even in software licensing, which means that costs are drastically low compared to conventional hosting. Salesforce.com is one of the best enterprise applications running on SaaS.
Compared to SaaS, utility computing is not new, but this sort of computing is backed by many sites like Amazon.com, Sun, and IBM who host servers on demand. Most companies who adopted it some years back used it for non-critical purposes but slowly and steadily, utility computing is bound to replace datacenters.
Web service providers offer APIs that help out developers to make the best use of its functionality online rather than creating full-blown applications. There are many companies which provide a full range of APIs like Google offers with its Google maps.
PaaS is a variation of SaaS where you can use the platform as a service. You build your own applications runs on the provider's platform. One does not get complete freedom here but one gets predictability and pre-integration.
MSP Managed service providers are the oldest forms of cloud computing. It is basically an application which manages IT and its applications rather than being useful directly to the end users. The best example of a MSP is a virus scanning service or even a monitoring service and anti-spam services. Even some desktop management services come under the canopy of managed services.