Spot Instances are spare EC2 compute capacity available at up to 90% discount compared to On-Demand prices, while a Spot Fleet is a collection of Spot Instances (and optionally On-Demand Instances) that launches and maintains capacity from multiple pools to optimize cost, performance, or capacity.
Spot Instances leverage AWS's unused EC2 capacity, offering significant cost savings (up to 90% off On-Demand prices) in exchange for potential interruption when AWS needs the capacity back. Spot Instances are ideal for fault-tolerant, stateless, or flexible workloads such as batch processing, big data analytics, CI/CD pipelines, containerized workloads, and development/test environments. Unlike On-Demand or Reserved Instances, Spot capacity can be reclaimed by AWS with a two-minute interruption notice.
A Spot Fleet is a collection of Spot Instances (and optionally On-Demand Instances) that launches and maintains a target capacity from multiple Spot Instance pools, each defined by instance type, Availability Zone, and price [citation:2]. Spot Fleet automatically requests capacity from the most cost-effective pools based on your allocation strategy, and can automatically replace instances that are interrupted or become unhealthy [citation:2].