AWS Lambda is an event-driven compute service; AWS Step Functions is a visual workflow service. The information that follows helps distinguish the two services and provides some of the considerations for determining which to use when.

Step Functions is a serverless orchestration service that lets you easily coordinate multiple Lambda functions into flexible workflows that are easy to debug and easy to change. Step Functions will keep your Lambda functions free of additional logic by triggering and tracking each step of your application for you. 

https://blog.clearscale.com/aws-step-functions-vs-aws-lambda/#:~:text=AWS%20Lambda%20is%20an%20event,determining%20which%20to%20use%20when.