The inventory screen pops up at the push of a button (keyboard, mouse, controller, touchpad, etc.) and then allows you to use, equip, or drop inventory items as necessary.
If you look down at your hands during "inventory/status mode" you will see a map of the dungeon level! This map is dynamically drawn to reveal only those areas of the level you have explored so far (although magical scrolls may reveal the complete map, or special features such as items, gold, or monsters)