Your UI, or User Interface, is the graphical organization of your charater frames, raid frames, and your bars/buffs/minimap; pretty much everything. I'm very A.D.D. with my UI, and it changed almost weekly until I found one that I have stuck with for a while.
If you go to www.wowinterface.com, there is an entire subcategory of downloadable UIs. My personal preference is to keep it minimalistic. I hide all my bars (even buffs like fort and inner fire, until I mouse over them), and keep things neat.
The current UI that I am using is called QuseUI, and it's available for download on www.wowinterface.com.
Another great resource if you're just starting off designing your UI is the Elitist Jerks SubForum - User Interface and Addons. Not only is there an extensive thread listing hundreds of downloadable UIs, but there is a UI help center, a thread dedicated to the art of interface design, and a WHAT'S THAT MOD??? thread if you're looking for something in particular to enhance your gaming experience.
If/when you decide to download a UI, installation is very easy. All you have to do is download the UI. Then, extract either the .rar or the .zip file to your desktop. One of two things should happen: One folder appears, with 2-3 folders inside. The first folder should be the title of the UI, and inside, the folders will be labeled Interface, WTF, and sometimes there is a folder labeled Fonts. Alternatively, the folders inside the original folder will appear on your desktop alone.
Open the WTF folder. Open the Account folder. Inside the Account folder, there should be one folder. Rename this folder, in all caps, your account name. Open this folder that is now labeled your account name. There should be two folders within this folder. Saved Variables is one of them, leave Saved Variables alone. Rename the other folder your realm name. If you realm name has two words, as mine does, make sure to add the space. Ex: 'Dark Iron'.
Open this (the realm) folder. There should be one folder inside this one. Rename this folder with your character's name.
At this point you can close all open folders, and open World of Warcraft. When you first log in, things are going to be all over the place. Don't panic!
Check back to the page where you downloaded the UI. Sometimes, if you're lucky, the author will post a command to align all of the addons as you saw in the picture.
If you're not so lucky, there should be a list of addons on the page. Open each individual addon in game. For example, if you see pitbull on the list of addons, type /pitbull in game. The pitbull options menu should come up. Select 'Profiles' on the left side of the menu, and go to the Copy From drop down menu. There should be a profile with a title that corresponds with the UI that you downloaded. Select this.
Rinse and repeat for all visible addons.
I hope this has helped some people with UI troubles. I'm pretty experienced with downloading/adjusting UI's, so if anyone has any questions, or maybe wants a few more available websites for downloaded, just contact me and I'll help ya out :)