Incremental Backup=A backup that copies only the files that have changed since the last backup.
Differential Backup=A backup that copies all files that have changed since the last full backup. Each differential backup will include all files in previous differential backups since the full backup so to restore a version of a file, you only need to search the full backup and the relevant differential backup.
It's been a while, so sorry I couldn't be a bit clearer. I used to know an easy way to remeber the difference between them all.
\"The surest way to corrupt a youth is to instruct him to hold in higher esteem those who think alike than those who think differently.\" Friedrich Nietzsche