Inventory menu
The inventory menu is a special in-game menu where the player normally, but not neccessarily, finds an inventory and other related stuff.
Controls
In the default configuration, the inventory menu can be opened and closed with I.
Refer to Inventory#Controls to see how to use inventories.
The default inventory menu consists of a 3×3 crafting grid with an output slot at the upper part and an 8×4 inventory (which is also refered to as the “player inventory”) at the lower part. The first line of the player inventory make the hotbar.
The default inventory menu is used in the vanilla Minetest game and in other games by default that do not specify a different inventory menu.
Other games
The inventory menu can be completely customized. Mods may add inventories and basicly every other available menu widges the modders like to. Mods can change the design of the menu and much more. In fact the inventory menu is really just a menu. The inventory menu can be designed to contain much more than just inventories or it can be designed to contain no inventories at all.