Reliability can be expressed in terms of your organization's Recovery Point Objective (RPO) and Recovery Time Objective (RTO) defined. RPO represents how much data you're willing to lose, and RTO represents how fast you can rebuild your infrastructure in the event of a failure.
Let's suppose that you run an e-commerce site. Around noon every weekday, you reach peak sales. Every time a user adds an item to their shopping cart, you store the items on a server-side cache so that users can resume their shopping spree later at home. In addition, you process hundreds of transactions per minute. Business is good, your infrastructure is scale-out beautifully, and everything is going smoothly. Meanwhile, a hungry rat or an overly charged lightning cloud decides to strike your data center. Initially, a seemingly harmless power unit goes down, but it...