In the middle starting with a Hotdog is the main bar that can be used for skills, items, shouts, etc. This is controlled by the normal numbers 1 to 0.

You can also expand the bar for more space on the left, but they do not have keys attached, so it requires a mouse click.

The second and third bar above the main bar you can use for skills, items, shouts, etc. They are controlled by Alt + number 1 to 0 and Ctrl + number 1 to 0.

At the top left corner, you see an additional bar, but they do not have keys attached, so it requires a mouse click, like the bar on the bottom left.

At the bottom left is the action slot, where you can drag and drop up to 5 skills. (You open the skills with the Hotkey-‘K’).

 You can activate it either by pressing the Hotkey-‘C’, by clicking on the ‘S’ symbol (left of the action slot) or by dragging the ‘S’ symbol into your main slot bar.