What is AWS Step Functions and when would you use it?

AWS Step Functions is a serverless orchestration service that lets you coordinate multiple AWS services (Lambda, DynamoDB, SQS, ECS) into visual workflows called state machines. State machines are defined in Amazon States Language (ASL) — a JSON-based definition of states (tasks, choices, parallel, wait, etc.) and transitions. When to use: (1) Multi-step business processes — order fulfillment (validate → charge → fulfill → notify); (2) Long-running workflows — processes exceeding Lambda's 15-minute limit; Step Functions can wait for callbacks (days/months); (3) Error handling and retries — built-in retry policies with exponential backoff per step; (4) Parallel processing — Fan-out to parallel branches, wait for all to complete; (5) Human approval workflows — pause and wait for an external signal (email approval); (6) Complex conditionals — Choice states with multiple branches. Step Functions replaces custom orchestration code with visual, auditable workflows. Standard Workflows bill per state transition ($0.025/1000); Express Workflows bill per execution duration.