For all intents and purposes, purple items are orange items that have their armoring mod locked and cannot be modified.
Not entirely true.
Purple items can also have inherent bonuses that are in addition to the mods. Check out the raid gear vendors on the station and look at the stats on the gear. For example one piece might give you +75 to Endurance, but 50 of that is from the mods, and the remaining 25 is inherent to the purple armor piece.