SaaS stands for Software as a Service.
SaaS is a model that aims to offer software application as a service which can be used for the purpose of software development and testing. SaaS testing takes place once the development process is over. The duration of testing cycle depends on the type of software opted as a service. SaaS is basically a model that delivers application over the web, managed by a third party vendor. The interface of a SaaS application is accessible by the end user over the web without the need to install it on the local machine. Few commonly used SaaS applications are - Salesforce, Cisco WebEx, Concur, Workday, Google Apps
PaaS stands for Platform as a Service.
In this, we get a framework to develop or customise our application. PaaS enables quick, simple and cost effective ways of developing, testing and deploying applications. With the help of third party provider, we can manage Operating systems, virtualization, storage, networking, servers and PaaS itself. 'Apprenda' is a provider of private cloud PaaS for .Net and Java applications.
In cloud testing, an application is tested on a web based platform than on local machines. Here we limit our discussion regarding testing of SaaS and PaaS applications. Cloud is quite popular in software development and testing.