Software as a service (SaaS) is a software application delivery model where a software vendor develops a web-native software application and hosts and operates the application for use by its customers over the Internet. Customers do not pay for owning the software itself but rather for using it.
Benefits of the model:
This is a new model but the uptake is accelerating. Look at www.salesforce.com for an example of how many customers are turning to this model.
Studies have shown that in the old model where companies built their own systems in house, 40% of developer's time was spent fixing bugs in live applications. In the Software as a Service model everything is taken care of by the vendor. Don't get caught in the old world.