The evolution or timeline of Lambda looks something like this:

On-Prem DataCenter –> IAAS –> PaaS –> containerization/Docker –> Serverless

Lambda is a compute service where you can upload your code and create a Lambda function. AWS Lambda takes care of provisioning and managing the servers that you use to run the code. You do not have to worry about operating systems, patching, scaling etc.

It is essentially described as an event-driven compute service where AWS Lambda runs your code in response to events. These events could be changes to the data in an Amazon S3 bucket or an Amazon Dynamo DB table.

Lambda events can trigger other Lambda events or call other AWS services like SQS or SNS.