Items can be obtained in many different ways, such as:
Indicates the category of item.
A boolean (yes / no) value indicating whether a given item can be placed on the map by a player.
Indicates the maximum Stack size for the item.
This table indicates the maximum stack of a given item.
Drops on the ground (i.e. not in your Inventory or a storage unit) upon logout of the save file will despawn (disappear). At that point, it is impossible to reacquire them. If there are more than 400 "loose" items on the world, items will also start despawning. In this instance, the items that have remained on the ground for the longest amount of time may vanish as new items are added to the screen.
In (possibly) both versions of the game, PC and Console, if you stand right in front of a 1 block wall and throw something, the item will go through the wall.