Explain exactly what AWS Lambda is, and for a specific use case, what did the Lambda function do?

AWS Lambda is a serverless compute service that lets you run code without provisioning or managing servers. You upload your code, and Lambda automatically scales it in response to incoming traffic. A common use case is image processing: when a user uploads an image to an S3 bucket, an S3 event triggers a Lambda function that resizes the image, stores the thumbnail back in S3, and updates a database record. The function runs for a short time, uses the event payload to determine the image key, performs the resize using a library like Pillow, and writes the result. This eliminates the need for a dedicated EC2 instance, reduces costs, and scales automatically.