S3 is object-based flat file unlimited storage. It's unlimited, but that doesn't mean we should throw files up there without thinking -- storage still costs money. It's not block-based, so it's not meant for storing operating systems or live databases. But any type of file can be stored (including database file backups), and each can be from 0 bytes up to 5 TB. General knowledge about S3 is one of the key categories in the AWS Certified Cloud Practitioner exam.