Difference between AWS Developer/SysOps tools

AWS CLI is a command-line interface to interact with AWS from the command-line, just like you would do using the AWS console UI - This can be used for creating resources as well as interacting with those resources. You would typically use this for administrative activities or shell script automation.

AWS SDK is a set of APIs in different supported languages like Java, Python, Node.js etc. using which you can programatically interact with AWS resources from your code - Although you can create resources using the SDK, you typically will use Infrastructure as Code such as Cloudformation or CDK to create resources and use SDK to interact with those resources such as putting an object in S3 or retrieving a message from an SQS queue.

AWS CDK is a programmatic way to do Infrastructure As Code. You can use YAML or JSON to write AWS Cloudformation templates to create infrastructure. CDK is a programming layer that builds Cloudformation templates for you under the covers, if you do not speak YAML or JSON, but would rather build your infrastructure using Java, Javascript, Python, Typescript etc. You will use this to create your resources in a repeatable and automated manner

There are also AWS plugins available in different popular IDEs such as VSCode, Eclipse, IntelliJ IDEA etc. that are open-source projects that make it easy to use AWS from your IDE of choice. If you are a developer using one of these IDEs, you can use the AWS toolkit for your IDE.