Hotbar

The Hotbar is the top row of your Inventory. Items placed in the hotbar can be used with a left mouse click. You can change the selected item by either using the scroll wheel, clicking the item on the hotbar, or pressing the associated number key.

As of 1.0.6 there is now an option to lock the hotbar. When the hotbar is locked, the selected item can not be changed by clicking. Also, hotbar slots now show their respective shortcut keys.