Cloud computing service model describe what types of service is provided to customer according to their business requirements. Cloud service provides different types of service, there are no hard and fast rules governing these service offerings. Most common services model are as below:

IaaS (Infrastructure as a Service)
PaaS (Platform as a Service)
SaaS (Software as a Service)

Infrastructure as a Service (IaaS):
Having the service user can get dynamic infrastructure facility by the provider in virtualized form. Billing amount is calculated as per unit resource usages like per CPU hour, data GB stored per hour, network bandwidth consumed, network infrastructure used per hour, value added services used, e.g., monitoring, auto-scaling etc. User can get high resource (CPU, Memory, and Network Bandwidth) for specific time duration. It serves as a foundation for the other two models, for their execution.
Amzon is the pioneer of IaaS. Other leading providers are Rackspace,Windows Azure,VirtuStream,, GoGrid, Joyent, Rightscale , NetMagic  and InstaCompute.

Platform as a Service (PaaS):
Service Provider provide cloud environment for development and deployment software. In this environment Application developer can do development to testing without buying complex hardware or can avoiding preparing and manage complex environment. With some PaaS offers, the underlying compute and storage resources scale automatically to match application demand such that the cloud user does not have to allocate resources manually.
With the model user can run web service, database, and development and testing tools.

SaaS (Software as a Service):
Cloud providers Prepare software deployment environment and user can host application software. Maintained and managed software and infrastructure by cloud provider. User can access work having the privilege which is assigned. He needs not to worry on taking backup, security or infrastructural maintenance. Most common SaaS providers are:
Project Management: DeskAway , Zoho Mail, Chat, Docs, Project, Sheet, Writer etc.
CRM apps – Impel CRM, Salesforce.com, and Microsoft Dynamics.
Cloud based Storage and Sharing services: Drop box, Skydive (windows live), Amazon S3, Google Docs, Box.net, Mozy.
SMEs/SMBs : EazeWork (for HR, PayRoll and Sales)