I don't believe that the Maintenance Plan wizard has a facility to schedule diff. backups.

>> What is the difference between a backup schedule and a maintenance plan that includes a backup?

When you schedule a backup, the SEM creates a TSQL jobs that executes BACKUP DATABASE. On the other hand, when you do a backup within a Maintenance Plan, all the work is done through an extended stored procedure ( master.dbo.xp_sqlmaint).