Differential backup
Also called: diff backup, differential
A differential backup copies only the files that have changed since the last full backup. To restore, you need exactly two pieces: that full backup plus one differential. Each differential grows over the week as more changes accumulate, but restore stays simple.
What this means
Copies only what's changed since the last full backup. To restore you need exactly two things: that full backup plus one differential. It grows through the week as changes pile up, but the restore stays simple, the full, then one differential on top.
Why it matters operationally
Differentials are the middle ground between full and incremental: cheaper to take than a full, simpler to restore than a long incremental chain. A weekly full + daily differential schedule keeps restores to just two backups.