Learning Mac Computers - Backup, Storage and Sharing
http://edu-observatory.org/olli/Mac_Computers/Week7.html   or   index.html

Having one or more backups is essential. 

  Sooner or later, you most likely will have something go
  wrong and lose your personal files, documents or photos.
  Examples include accidentally deleting the wrong files,
  hardware failure, losing your laptop or infecting your
  computer. At times like these, backups are often the only
  way you can rebuild your digital life.

  OUCH! -- Backup & Recovery

Backup Options

Use Time Machine to back up or restore your Mac


  OS X El Capitan: Time Machine options preferences

  OS X El Capitan: Connect a new backup disk to your Mac

Backup to external Hard Drive or Flash via command line
  rsync -av -e ssh /Users/swormley /Volumes/My\ Passport
  rsync --delete -av -e ssh /Users/swormley /Volumes/My\ Passport

Backup to another computer on LAN via command line
  rsync -av -e ssh `pwd`/  mm3.local:`pwd`

Recall Dropbox (Free service for all platforms)

  Dropbox is a file hosting service operated by Dropbox, Inc.,
  headquartered in San Francisco, California, that offers
  cloud storage, file synchronization, and client software.
  Dropbox allows users to create a special folder on each of
  their computers, which Dropbox then synchronizes so that it
  appears to be the same folder (with the same contents)
  regardless of which computer is used to view it. Files
  placed in this folder also are accessible through a website
  and mobile phone applications.

  One can backup specified files and folders to Dropbox using
  symbolic links via the command line. For example:
  ln -is ~/Documents ~/Dropbox/Documents

  Anything added, deleted or changed on the Mac is replicated
  in Dropbox and vice versa. Deleting File/Folder in Dropbox
  also deletes it immediately from the Mac! Just so you know.

Using The Cloud Securely

iCloud Backup & Storage

Carbonite (backup in the cloud)
Crashplan (backup in the cloud)