The Inventory tab of the PDA has two sub-tabs: the Inventory tab and the Cargo tab.
The Inventory tab lists all of your possessions in the universe, sorted by location. While you can view anything from this tab, you cannot perform any actions on your belongings (e.g., selling them or having them delivered).
The Cargo tab lists the cargo currently in your ship. It can be brought up in space by pressing the "j" key. If invoked from in space, this screen will allow the player to jettison anything from his cargo hold, making it a very important screen when mining.