The following explanation of differential and incremental backups applies specifically to 3Essentials backup solutions, and should apply generally to most discussions of incremental versus differential backup strategies, although backup terminology can vary based on backup technology, vendor and solution. 

A weekly Full/Differential backup strategy has two parts:

• A weekly FULL backup, which backs up all the files on the system. 
• Each of the remaining six days of the week, a differential backup is run. 
• A differential backup backs up files that have been modified since the last full backup.  So...
  • Monday's differential backs up everything since changed Sunday's full backup (changes on monday)
  • Tuesday's differential backs up everything changed since Sunday's full backup (changes on monday and tuesday)
  • Wednseday's differential backs up everything changed since Sunday's full backup (changes on monday, tuesday, and wednesday)
  • etc...

Positives associated with the differential approach: 

• Lower risk: To restore data back to the way it was on Thursday , for example, you only need Sunday's FULL backup, and Thursday's DIFF backup.  Those two backup sets together will contain all the files as they sat on the system on Thursday.  This means a failure of Wednesday night's backup will not impact your ability to restore data to date and time of Thursday.  
• As a result of this, the differential approach is lower risk. 

Negatives associated with the differential approach: 

• Significantly more backup space required.  If you look at the chart above, each day's differential will contain files that the previous day's differential contains.  For example, consider some file "myfile.txt", which is modified on Tuesday only:
  • will be included in Sunday night's full backup, as all files are backed up in the full. 
  • will not show up in Monday night's backup, because it wasn't modified on Monday. 
  • will show up in Tuesday night's differential backup, becuase it was modified since the last full backup.
  • will show up in Wednesday night's differential backup, becuase it was modified since the last full backup.
  • will show up in Thursday night's differential backup, becuase it was modified since the last full backup.
  • will show up in Friday night's differential backup, becuase it was modified since the last full backup.
  • will show up in Saturday night's differential backup, becuase it was modified since the last full backup.
  • At the end of the week, you will have backed up that file 5 times... making your backup space requirements for just that single modified file 5 times larger than the incremental backup.  
• As a result of this, the differential approach requires considerably more backup space. 

A weekly Full/Incremental backup strategy has two parts:

• A weekly FULL backup, which backs up all the files on the system. 
• Each of the remaining six days of the week, an incremental backup is run. 
• An incremental backup backs up files that have been modified since the last full OR incremental backup.   So... 
  • Monday's incremental backs up everything since changed Sunday's full backup (changes on monday)
  • Tuesday's incremental backs up everything changed since Monday's incremental (changes on tuesday only)
  • Wednseday's incremental backs up everything changed since Tuesday's incremental (changes on wednesday only)
  • etc...

• Significantly reduced backup space required.  If you look at the chart above, each day's differential will contain files that the previous day's differential contains.  For example, consider some file "myfile.txt", which is modified on Tuesday only:
  • will be included in Sunday night's full backup, as all files are backed up in the full. 
  • will not show up in Monday night's backup, because it wasn't modified on Monday. 
  • will show up in Tuesday night's incremental backup, becuase it modified since the last full OR incremental backup.
  • will not show up in Wednesday night's incremental backup, becuase it was not modified since the last full OR incremental backup.
  • will not show up in Thursday night's incremental backup, becuase it was not modified since the last full OR incremental backup.
  • will not show up in Friday night's incremental backup, becuase it was not modified since the last full OR incremental backup.
  • will not show up in Saturday night's incremental backup, becuase it was not modified since the last full OR incremental backup.
  • At the end of the week, you will have backed up that file only 1 time in the incrementals.... making your backup space requirements for just that single modified file significantly less than if you used the differential approach. 
• As a result of this, the incremental approach requires considerably more backup space. 

Negatives associated with the incremental approach: 

• Slightly higher risk: To restore data back to the way it was on Thursday for example, you only need Sunday's FULL backup, plus the incrementals from each of Monday, Tuesday, Wednesday and Thursday:
  • ​You start by restoring the FULL, which is all the files as of Sunday.  
  • And then restore from Monday's incremental in order to restore all the changes from Sun - Mon.  
  • Then restore Tuesday's incremental, in order to restore the changes from Mon - Tue
  • and etc up to Thursday.  
• This doesn't slow down the restore process at all; indeed the backup software handles restoring all of those incrementals simultaenously and sorts out getting the right version of the file in place if it was changed multiple times through the week.  But the risk is that you need all of the incrementals between the last full backup, and the date you want to restore to.  If one of those incrementals backup media experienced a corruption or failure,  then your restore wouldn't necessarily have all the files as they existed on Thursday.  For those files, the most recent copy you might have would be from Sunday's full, but for the files changed on the date of the incremental that failed, you would not have those modified versions of those files.  
• While we do monitor for backup job failures, and our backup solution does perform a verify of backup sets upon backup completion, but backup file/media corruption can and does occur... they are simply files on a file system, just the like the files you're wanting backed up.