Consider a vault like your traditional file cabinet. It even has an icon that resembles the file cabinet in the SmartVault client portal. You can create as many vaults as you like. Once you give someone access to it, they get access to all of the folders within it. If you had new folders the user has access to those as well. Vaults are shared using group membership.
A vault contains folders. It can also contain special type of folder, Applications. You can has as many folders in the vault as you want.