In the domain of SQL Server backup, what is a backup set, backup device, backup media, backup media set, backup media family
The screenshots from SSMS will help you understand the terms:

backup set - is a backup from a single, successful backup operation. E.g. a full database backup created at 8 AM yesterday


You can see that there are 2 backup sets in a single backup media set

backup device - a disk file or tape drive

backup media - tapes or disks where backup are stored
backup media set - in the screenshot - it's the BAK file where backups are added



backup media family - You're creating the 8 AM full database backup and your database is huge, the backup takes more space than 4 tapes can store. Say you have 4 tape devices that are used to backup your database. When one tape is full, you insert a new one. If your database backup takes 20 tapes, that's 5 tapes per each tape device. The 5 tapes created on the tape device 1 belong to the same backup media family, as they have the same "father" - tape device 1