"Here’s the problem: Elasticity and scalability should never be used interchangeably. Small- and mid-sized business (SMB) leaders need to know that while elasticity isn’t a must-have for every SMB, scalability is. The same goes for most enterprise businesses as well.
Elasticity is aimed at companies building consumer- or business-facing software applications that they plan to sell on a subscription basis. Think: Evernote, Netflix, Dropbox, and Salesforce.com. Elasticity basically means that your platform can handle sudden, unanticipated, and extraordinary loads. This could be the result of a Superbowl ad or some other widespread promotional technique that results in a massive but brief influx of users and load on the system.
Think of elasticity as, essentially, unlimited head room. When you’re a software developer building SaaS that you plan to offer to the entire planet (such as Facebook, which hopes to have the whole world as users), you need unlimited head room for those unpredictable moments.
Contrast that to scalability. Scalability is a planned level of capacity, with appropriate overhead, that you anticipate your company’s systems to require over time, in addition to the ability to scale in a quick and easy manner when (and if) you need more (or less) resources."