Item
An item is an object that exists only within the player's inventory and hands, or displayed in item frames, glow item frames, or armor stands.1
Behavior
Some items, when used, place a block (ItemBlock) or entity (minecart, spawn eggs, etc.) version of themselves into the game world. Put simply, they are an item when in the inventory, and a block when placed. For example, boats turn into an entity when placed, and beds turn into a group of blocks when placed. When selected in the hotbar, items briefly display their names above the HUD.
The only method by which an item can be properly displayed within the game environment is to place it into an item frame.
If an item that does not become a block is dropped, it becomes an entity represented by a sprite that floats above the ground for 5 minutes in a loaded chunk before despawning, unless the player walks over it to pick it up before it despawns, it is picked up by a mob, hopper or minecart with hopper, or it is destroyed by fire, lava, cactus, or explosions.
A submerged object ascends towards the (water’s edge. When the surface current is in motion, the object is propelled along with it.
Hoppers draw in any items that are placed above them.
Most items stack to 64, but some only stack to 16 and others not at all but all types of items can be stacked up to 127 through inventory editing.1
List of items
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Source: Minecraft Wiki. ↩↩