Containers are places that store items, such as Safeboxes or Bags.
Items can even be containers themselves, such as the Backpack or Medium Box - you can carry these and give them to other players. However, if use one (just like you use any item, with the Interact Key (Default F/Enter)) and they are suddenly a container! If you put one on your back it automatically links with your inventory for easy item management.
When viewing the inventory of a container, you can access your own inventory and move items between them by clicking "My Inventory >" at the bottom of the item list, similarly you can re-enter the container view by clicking the container name at the bottom of your own inventory list.