Cloud computing, often referred as "cloud", is the delivery of storing and accessing the data and programs over the Internet from anywhere in the world. There are three models of Cloud Computing. IaaS, PaaS & SaaS
SaaS (Software as a Service)
Cloud application services, or Software as a Service (SaaS) uses the web to deliver applications that are managed by a third-party vendor and whose interface is accessed on the clients side. Most SaaS applications can be run directly from a web browser without any downloads or installations required, although some require plugins.
Because of the web delivery model, SaaS eliminates the need to install and run applications on individual computers. With SaaS, it’s easy for enterprises to streamline their maintenance and support, because everything can be managed by vendors: applications, runtime, data, middleware, servers, storage and networking.
Examples: Google Apps, Salesforce, Workday, Concur, Citrix GoToMeeting, Cisco WebEx
PaaS (Platform as a Service)
Cloud platform services, or Platform as a Service (PaaS), are used for applications, and other development, while providing cloud components to software. PaaS makes the development, testing, and deployment of applications quick, simple, and cost-effective. With this technology, enterprise operations, or a third-party provider, can manage servers, storage, networking, and the PaaS software itself. Developers, however, manage the applications.
Enterprise PaaS provides line of business software developers a self-service portal for managing computing infrastructure from centralized IT operations and the platforms that are installed on top of the hardware. The enterprise PaaS can be delivered through a hybrid model that uses both public IaaS and on-premise infrastructure or as a pure private PaaS that only uses the latter.
Applications using PaaS inherit cloud characteristic such as scalability, high-availability, multi-tenancy, SaaS enablement, and more. Enterprises benefit from PaaS because it reduces the amount of coding necessary, automates business policy, and helps migrate apps to hybrid model.
Examples: AWS Elastic Beanstalk, Windows Azure, Force.com, Google App Engine, Apache Stratos
IaaS (Infrastructure as a Service)
Cloud infrastructure services, known as Infrastructure as a Service (IaaS), are self-service models for accessing, monitoring, and managing remote datacenter infrastructures, such as compute (virtualized or bare metal), storage, networking, and networking services (e.g. firewalls). Instead of having to purchase hardware outright, users can purchase IaaS based on consumption, similar to electricity or other utility billing.
Compared to SaaS and PaaS, IaaS users are responsible for managing applications, data, runtime, middleware, and OSes. Providers still manage virtualization, servers, hard drives, storage, and networking. Many IaaS providers now offer databases, messaging queues, and other services above the virtualization layer as well. Some tech analysts draw a distinction here and use the IaaS+ moniker for these other options. What users gain with IaaS is infrastructure on top of which they can install any required platform. Users are responsible for updating these if new versions are released.
Examples: Amazon Web Services (AWS), Cisco Metapod, Microsoft Azure, Google Compute Engine (GCE)