Amazon Elastic Container Service (ECS) is a container orchestration platform which can run containers on EC2 instances or serverless using Fargate. For many container-based systems we find ECS to be a good choice thanks to its simplicity, maturity and basic but competent feature set.
Amazon Elastic Kubernetes Service (EKS) is:
AWS Step Functions is a serverless function orchestrator that uses a state machine to tie together Lambda functions and other supported AWS services to build complex serverless workflows. Step Functions can integrate directly with many AWS services such as Lambda, DynamoDB, SQS, SNS, API Gateway, SageMaker and Athena. A state machine can be deployed, started and monitored through the console or an AWS SDK.
Kubernetes is an open source container orchestration platform, highly regarded for its flexibility and scalability. While Kubernetes is often considered the de facto standard for container orchestration, we recommend careful consideration before adopting due to its operational complexity. In many cases, native cloud platform orchestration options such as ECS may prove to be a better fit, especially for relatively small or simple deployments.