A full backup is where you copy all your data from it’s existing location to a backup location. When running frequent backups on the same data (to keep you backups up to date), it will take the same amount of disk space and time to make a new full backup every time. To avoid this, a differential backup can be used. A differential backup only copies the difference or change in data since the last backup was run.
For example, say you run a full backup on 1000 files, and you then edit 2 of these files. If you now run a differential backup, it will only contain the 2 files that have changed. This means that 2 files (rather than 1000 files) need to be copied, saving a lot of disk space and time.
The downside of this approach is that as the full backup gets older, the amount of data which changes since that full backup was run increases, so each new differential backup gets bigger. To avoid this, incremental backups can be used instead.