the PVP locker is sort of like a bank for your SPVP skins you've unlocked on your account. you can put old sets in there and take them out agian later if you decide you want to wear that look. The bank has a slot for each armor skin avalible through sPVP so you never have to choose between space and keeping a desired skin.
Cultural armor is a special armor skin unique to a particular race, they can be purchased in the home city of the race from vendors. I think the first tier of cultural armor is at level 60 but I could be mistakin on that one.