Scalability is a measure of how an application would react when the resources at its disposal are scaled up. If an application supports 10,000 users with a given infrastructure, can it support at least 20,000 users with double the infrastructure?

If a web application does not respond within a couple of seconds, there is a very high chance that the user of your application will be disappointed.

In the world of the Cloud, the scalability of applications becomes even more important. It's difficult to guess how successful a start-up might be. Twitter or Facebook might not have expected such success when they were incubated. Their success, for a large measure, depends on how they were able to adapt to a multifold increase in their user base, without their performances being affected.