Just like a bucket holds water, Amazon buckets are like a container for your files. You can name your buckets the way you like but it should be unique across the Amazon system.
Cloud front :
In the case of S3, you were only paying for data served from S3 to the end user but in case of CloudFront, you also need to pay for data transferred from S3 to these edge locations.
it also explaing costing related benefits.