Full Backup : Backs up all the data pages.
Differential Backup : Backs up all the extents which have been modified since the last full backup.
Log backup : A transaction log backup allows you to backup the active part of the transaction log. So after you issue a "Full" or "Differential" Log backup can only work when the transaction logrecovery model is Full/Bulk-logged and there has been a full backup will have anytaken . It has all transactions that were created after those other backups completedlogged and is a very important part of point in time recovery.