All great (and exam-specific) points by SmartCoder. If I may add a general ancillary comment:
Wittgenstein said, "In most cases, the meaning of a word is its use." I think this maxim is remarkably apt for software engineering too. In the context of your question, those two AWS services are used for significantly different purposes.
Lambda - Say you developed a photo uploading application with Node.js that uploads some processed images to an S3 bucket. The core logic for this is probably quite straightforward, and it's got a singular, distinct task. Simply take in an image, do some processing and if not for any exception, store it in a bucket. In this case, it's inefficient to waste time spinning up servers, configuring them with a runtime environment, downloading dependencies, maintenance, etc. A literal copy and paste of your code into the Lambda console while setting up a few configurations should get your job done. Plus, you save a lot of money as infrastructure is "provisioned" only when your Node.js function is invoked. Again, keep in mind the principle of this code performing a singular task.
Elastic Beanstalk - This same photo uploading system mentioned above might now mature into a more complex full-fledged software application that requires user management, authentication, and further processing of the images, which certainly requires more provisioning of resources. This application will probably do a lot of things with multiple code repositories for you to manage and deploy. And yet, you don't want to spend money on a DevOps engineer or learn to use an IaC (Infrastructure as Code) platform like CloudFormation or Terraform. In this case, Elastic Beanstalk is useful for a developer without too much in-depth DevOps knowledge as it's a PaaS (Platform as a Service) tool; it pretty much gives you a clear interface to spin up whole new production-ready systems.
Here are two good whitepapers I read a while back on the above topics.
https://docs.aws.amazon.com/whitepapers/latest/serverless-architectures-lambda/serverless-architectures-lambda.pdf
https://docs.aws.amazon.com/whitepapers/latest/introduction-devops-aws/introduction-devops-aws.pdf