The belt interface in Diablo III is an evolutionary improvement on the versions seen in Diablo I and Diablo II. The form and function will be familiar to players experienced in the other games, but there are a few major changes in Diablo III that make the controls easier to use, but also guide players to operate the game as the D3 Team intends.
The look of the belt interface is similar to what we saw in Diablo II, with a few changes. Realize that this form is subject to further change during the ongoing development process.
The most recent version of the belt interface was revealed in April 2009.
This screenshot, of a Barbarian's belt interface, shows the key features.
There are 5 hotkey slots, into which skill icons or potions can be dragged. Hitting that button will immediately activate that skill or drink that potion. This is something of a change from Diablo II, where skills were always mapped to the mouse buttons, and could only be activated by clicking the hot key to move them to the mouse click. The 1-5 hotkeys in Diablo III work like the belt slots in Diablo II; for example Identify or Town Portal scrolls could be placed there and cast by clicking the number.
The left click and right click slots are marked by an appropriate little mouse icon. The "Tab" key (and the mouse wheel) switches between two active skills on the right click. Skills are added to these controls by clicking them in the skill tree, and they can then be cast immediately with a left or right click.
The six icons seen to the far right are various game controls, though it's not yet known exactly what function each provides. They're enlarged and brightened in this thumbnail, for easier study.