A Spot Instance is an unused EC2 instance that is available for less than the On-Demand price. Because Spot Instances enable you to request unused EC2 instances at steep discounts, you can lower your Amazon EC2 costs significantly. The hourly price for a Spot Instance is called a Spot price. Your Spot Instance runs whenever capacity is available and the maximum price per hour for your request exceeds the Spot price.
Spot Instances are well-suited for data analysis, batch jobs, background processing, and optional tasks.