IaaS offers the basic infrastructure required to setup a computing cloud such as storage, computing power, network, etc. the end user is still required to do substantial tasks such as manual configuration of the network, storage space, OS, resource management etc.
PaaS offers basic hardware as well as software services that allows end users to get straight down to business of developing and testing applications. PaaS providers might also offer automatic resource management service for automatic scaling according to additional need for computing power, storage, etc.
Software that is deployed over the internet. With SaaS, a provider licenses an application to customers either as a service on demand, through a subscription, in a “pay-as-you-go” model, or (increasingly) at no charge when there is opportunity to generate revenue from streams other than the user, such as from advertisement or user list sales.
SaaS - Software as a Service. Basically, this means applications.
PaaS - Platform as a Service. Includes operating environment and allows for deployment of application services. Example: Microsoft Azure.
IaaS - Infrastructure as a Service. A self-service model for accessing, monitoring, and managing remote datacenter infrastructures, such as compute (virtualized or bare metal), storage, networking, and networking services. Example: Amazon EC2