An S3 bucket is a storage container used in Amazon S3 (Simple Storage Service) to hold data in the cloud. Each bucket can store an unlimited amount of data and can contain multiple files, known as objects. Users can organize these objects using a hierarchical structure of folders and subfolders, making it easy to manage large datasets. Buckets are accessible via the internet, allowing users to upload, download, and share files securely. They can be configured with various permissions to control who can access the data, ensuring that sensitive information remains protected while still being available to authorized users.