ECS ( Elastic Container Service ) was the first container orchestration tool offering by AWS. It essentially consists of EC2 instances which have docker already installed, and run a Docker container which talks with AWS backend
Fargate was the second service offering to come, and is intended to abstract all everything bellow the container (EC2 instances where they run) from you. In other words, a pure Container-as-a-Service, where you do not care where that container runs.
