Amazon EC2 Reserved Instances and Other AWS Service Reservation Models
AWS Whitepaper

Amazon EC2 Reserved Instances

When you purchase Reserved Instances, you make a one-year or three-year commitment and receive a billing discount of up to 75 percent. For the right workloads, Reserved Instances can save you a lot of money.

A Reserved Instance is not a physical instance. It is a billing discount applied to the use of On-Demand Instances in your account. These On-Demand Instances must match certain attributes to benefit from the billing discount. You pay for the entire term of a Reserved Instance, regardless of actual usage, so your cost savings are closely tied to use. Therefore, it pays to plan and monitor your usage to make the most of your investment.

When you purchase a Reserved Instance in a specific Availability Zone, it provides a capacity reservation. This improves the likelihood that the compute capacity you need is available in a specific Availability Zone when you need it. A Reserved Instance purchased for an AWS Region does not provide capacity reservation.