Differential backup (also called cumulative incremental backups) scheme starts with the first full copy. The second one contains only the data changed since the last performed full backup - just like an incremental backup. But when it comes to the third and following copies, they will consist only of changes but always in relation to the last full copy, oppositely to an incremental backup where the following copy compares data with a previously done incremental backup.
Differential backup requires more storage and in most cases, longer lengths of time to complete than the incremental backup approach. The rule here is simple - the further from the last full copy, the bigger the copies are. However, differential backup enables faster recovery because only two backup sets are needed to restore your assets (last differential backup and the last full copy).