Here’s Amazon’s description of Standard-IA:
Amazon S3 Standard – Infrequent Access (Standard – IA) is an Amazon S3 storage class for data that is accessed less frequently, but requires rapid access when needed. Standard – IA offers the high durability, throughput, and low latency of Amazon S3 Standard, with a low per GB storage price and per GB retrieval fee. This combination of low cost and high performance make Standard – IA ideal for long-term storage, backups, and as a data store for disaster recovery. The Standard – IA storage class is set at the object level and can exist in the same bucket as Standard, allowing you to use lifecycle policies to automatically transition objects between storage classes without any application changes.