Also called: diff backup, differential
In plain English
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.
Why it matters
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.