All of your possessions are stored in your locker. Even items which have no physical representation.
Clothing
Upgrades
Themes
It is a place you can browse your possessions, and when you receive new items (through Email, purchase, Marketplace, etc.) they show up here as well.
You can duplicate most possessions but most possessions must be "refurbished" before trading. This is done by editing the item from within your locker and choosing "Refurbish".