Q: Back-Up - In SIMPLE terms, please...

Most things are included in backups. Look here for information: About backups for iOS devices - Apple Support

Actually, videos and music are not included in a backup, just the information as to which ones you have on the device. All iTunes media is re-downloaded to your device when you restore from a backup.

Also, all iCloud sync'd info is not backed up - Contacts, Calendars, Mail, Notes, Photo Stream, etc., and all iCloud Services are not backed up: iCloud Photo Library, iCloud Music Library, iCloud Drive.

Camera Roll is backed up as well as Messages, Settings, Homescreen Layout, App data, and Ringtones:

iCloud: iCloud storage and backup overview

Cheers,

GB