Web 2.0 has highlighted the possibility of using a simple browser as a client for any application. If we add to this the fact that broadband reaches a large part of the population, it is clear that it is possible to use an application without the need to have the hardware and software necessary to run it and without having to manage all the related problems, and related costs, which accompany them. From these technological assumptions a consolidated model emerged which is defined below.

Software as a service (SaaS) is a model for the distribution of software where customers access software over the Internet using a standard browser. Software as a service (SaaS) is centrally hosted and generally is licensed on a subscription basis.

The SaaS does not need to install and run applications on the infrastructure (HD and SW) of customers, thus eliminating the problems and costs associated with the maintenance and support of the software, operating system, and hardware. In return, customers are forced to forgo control over software versions or the ability to change their requirements. The most significant change is however represented by the payment method that from a significant initial expense turns into a periodic fee. The great innovation of SaaS providers is that they have modified the pricing model by basing it on monthly fees or on actual consumption. This has allowed companies to free from the fixed and continuous cost of licenses, allowing them to buy access to applications and data when they actually need it or rather “on-demand”.
The SaaS also allows software vendors to control and limit the use of the software and therefore prohibit illegal copies and manage the different derivative versions. This centralized control makes it possible to obtain a continuous and more easily estimable income.
In the following figure it is possible to compare the classic licensed model and the new SaaS model.

Many software vendors do not host their SaaS applications on proprietary web servers but rely on third parties to reduce their hardware investments and focus on their core business, software development. In these scenarios the basic SaaS model is extended to include Cloud Providers. These subjects are able to lower the costs of deployment, management and support of the infrastructure by exploiting economies of scale.

The following figure shows the evolved model of SaaS which includes Cloud Providers.

Main features

The software supplied as a service by a vendor generally has the following features:

• Multi-tenant architecture: all users and applications share a unique and shared infrastructure and code base, maintained centrally. Because all customers of a SaaS vendor are on a single infrastructure and code base, vendors are able to implement innovations more quickly and save time that was previously used to maintain various versions of obsolete code. The pricing and customer management policies are also one-to-many.
• On-demand Pricing model: The license-based pricing model abounds in favor of consumer pricing models or periodic fee models. The service includes maintenance, support and updates.
• Web Interface: The only infrastructure requirements required are a simple web browser and a broadband connection. The use of the familiar internet interface also allows reducing the learning curve of a new software favoring its adoption.
• Data storage is entrusted to the SaaS vendor: The SaaS provider is also responsible for data storage and security.
• Benefits of the network: A SaaS solution in addition to its functionality can provide additional value to customers if it can leverage other SaaS solutions. Using the web as a platform provides the ability to create mashups or use plugins from other applications.