To put it simply for beginners:

Both IAM Users and IAM Roles can be used to access AWS services, but the key difference is that IAM Roles provide temporary credentials that are automatically managed and rotated, while IAM Users have permanent credentials (access keys and passwords). You can think of an IAM Role like a temporary API key that your EC2 instance or Lambda function can assume, whereas an IAM User is like a permanent API key tied to a person or app.