Inventory is the place where the items you currently have on you are displayed. To access it, press I; to move around it, use the scroll wheel.
Use G to "grab" an item from the ground, or Control+G to pick up a whole pile of items at once.
Shift+Click an item to drop it on the ground from inventory without having to drag it.
Use Control+D to drop everything in your inventory all at once into a single hex.
While accessing a container with items, use Control+Click on items to move them quickly without having to drag them.
Use Steal skill on an open container to access its contents, and use the "Put All" or "Take All" buttons as needed.