SAN's communicate using the SCSI protocol (but not infrastructure, fibre remember
). When you added a drive to a server it would give it a unique number so if any commands came through it would know which drive you meant.This number was the LUN, and today when you say LUN people mean the actual volume because it doesn't always relate to a single disk any more it's just where the server is supposed to direct things - a target.
I have SCSI Disk Drives in this PC i'm using and it does this upon boot. It will only address a set number of drives and they must have different identifiers associated because it uses these for communication with the drives.
I have two SCSI drives so it has given them numbers 0 and 1. 0 represents the whole storage of one drive and 1 the other.
When it comes to SANs the array knows which drives the data is destined for because the server just sees one volume and sends it everything at the one LUN number it has associated with that volume and in the background the array works out what goes where.
So going into detail - a LUN is a number a server throws data at. The array provides a 'target' (SCSI target) for the data.
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