This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.
Backup software usually provides some form of management of files, tapes, and autochangers. Retrieval of files is not automatic (as in true HSM). These are designed to allow you to recover from disk or file system failures, and to recover files accidentally (or maliciously) deleted or corrupted. Some work in conjunction with HSM systems, which are often vulnerable to the latter class of problems. I've concentrated here on backup software that supports various autochangers, as this is of more interest to people in this group than standalone software for backing up one hard disk onto one tape.