are they a piece of storage? same function?
They are of storage terms only.
raw is the amount of actual HDD disk space like for eg: If you have two hard disks of 100 GB each then raw capacity is 200 GB
A logical unit number (LUN) is a unique identifier used to designate individual or collections of hard disk devices for address by a protocol associated with a SCSI, iSCSI, Fibre Channel (FC) or similar interface.
LUNs are central to the management of block storage arrays shared over a storage area network (SAN)