Difference b/w IAM role and IAM user & how they together solve real time scenarios?

An IAM role is assigned to AWS resource like EC2, which allows access to other AWS service an IAM user instead is a user (other than root user) which is granted permission for use of respective AWS service or services.

So, suppose in the above example if an EC2 instance is assigned an IAM policy to access S3 bucket then the respective IAM AWS user 

should also have access to S3 bucket.