What is Amazon S3?

Amazon S3 (Simple Storage Service) is an object storage service offering industry-leading scalability, data availability, security, and performance. It stores data as objects (files) in buckets (containers), accessible via a unique URL. Key features: unlimited storage capacity; objects can be 0 bytes to 5TB (5GB per single PUT, use multipart for larger); 99.999999999% (11 nines) durability — data replicated across minimum 3 AZs; 99.99% availability; global access via HTTP. Bucket naming: globally unique, lowercase, 3-63 characters, no periods (for SSL). URLs: https://bucket-name.s3.region.amazonaws.com/key. Storage classes: S3 Standard (frequent access); S3 Intelligent-Tiering (automatic cost optimization); S3 Standard-IA (infrequent access, cheaper storage but retrieval cost); S3 One Zone-IA (single AZ, cheaper); S3 Glacier Instant Retrieval, Glacier Flexible Retrieval (archival, minutes to hours retrieval); S3 Glacier Deep Archive (cheapest, 12h retrieval). Key features: versioning (keep multiple versions), lifecycle policies (auto-transition to cheaper class or delete), replication (cross-region CRR, same-region SRR), static website hosting, pre-signed URLs (temporary access without auth), access control (bucket policies, ACLs, IAM), S3 Object Lock (WORM compliance), event notifications (trigger Lambda on upload), Transfer Acceleration (CloudFront edge for faster global uploads).