The information displayed on the center button is configurable. The metered bar around it could be a graphical display of your health, while the numerical display is your percent and amount of mana. These are reversible in the settings. The button itself has a variety of selectable actions, depending on your preference - I have mine set up to switch me in and out of Ghost Wolf form.
The top button auto-detects your potions. Left-click for a health-pot, right click for a mana pot. The button to the right of that auto-detects food in your bags, left click for food, right click for drink.
Next up, moving clock-wise is your weapon buff button. Click it to cast the shown weapon buff. Click it again to cast on your second weapon (if you're a dual-wielding monster of destruction). Right-click to expand and select a new weapon buff.
The two buttons in the bottom-right area of the wheel are your Action buttons. These two buttons are completely configurable through the settings menu, so you can select both a left and right click for them. I'm a fan of remove poison and cure disease for these slots.
The bottom button is your mount/hearthstone button, and it does just that.
Each individual button for the totems (those 4 on the left) lets you click to drop the displayed totem, middle-click to drop the last totem you dropped before the current choice, and right click will expand the button showing all totems of that type (Earth, Fire, Wind, Water).
On top of the wheel, Totemus also includes totem timers, in the form of a short bar with 4 icons. As you drop totems, the totem icon is shown in that element's location, with a countdown timer underneath it. Great for seeing when your Strength of Earth is going to expire.
The only downside of Totemus, for me, is that the primary actions of each button aren't bindable to keys. I'd love to be able to set my totems to Alt-1 through Alt-4. As it is, I've had to setup another Bongos bar for this action, almost negating the benefits of the built-in Totem control of Totemus.
Still, as a new shaman it's hard to NOT recommend this mod. If you, like me, found totems a bit overwhelming at first, and lamenting at the lack of action bar buttons to place all your weapon buffs and tons of totems, then you'll really appreciate what this simple mod can do for you.
Totemus can be downloaded at: http://wow-en.curse-gaming.com/files/details/5723/totemus/
Are you using Totemus? Let us know your experiences with the mod in the comments!