Whether you call them addons, macros, mods or scripts, WoW UI mods are all designed to change the user interface for World of Warcraft. The UI can be customized in all sorts of ways and this is the kind of thing that Blizzard encourages developers to due because it makes the game better for players. If you're looking for a way to modify your WoW gaming experience, here's some options you might consider.
Instead of just looking at individual World of Warcraft addons, I thought it might be more useful to consider some of the collections of addons available as they usually come with a whole range of useful mods that you can use and are tidily available in a single download.
Cosmos is one popular library of mods for World of Warcraft. It has some useful toolbar Addons and you can use Cosmos to configure the WoW UI to your liking. Using Cosmos will also help you cut down on the use of slash commands. All of the mods are configurable within Khaos (a configuration menu). Most of the Addons in the Cosmos library are designed to increase UI functionality. You can get Cosmos at Cosmosui.org and they include a complete list of all the addons in its library.
CTMOd is another popular collection of WoW Addons that you can install in either a basic installation or an advanced installation. If you're going to use CTMOd, you might as well just download the advanced zip file as it features a lot more mods. Some favorites are CT_RaidAssist, which will provide notifications for hp, mana, buffs and debuffs, etc. CT_UnitFrames lets you customize the appearance of the unit frames in WoW. CT_ExpenseHistory is great for tracking what you are spending on repairs, ammunition, and reagents, etc. You can take a look at the rest of the CTMod library on ctmod.net.
Titan Panel might be my favorite of the addon libraries for WoW. The horizontal bar can be displayed at the top or bottom (or both!) of you interface and it comes with an installed set of plugins that work with it. There are also a bunch of other plugins that can be used with it. Curse Gaming is probably the best source for getting Titan Panel. I would download and install it and see if its default set of plugins work well for you and then if you want to add some additional ones, see what's available.
How to Install WoW Addon Suites
Most of the addon suites for World of Warcraft will be downloadable as a zip file. Once downloaded, just unzip the files into your Addons directory. This is inside the Interface folder of your World of Warcraft installation.
Just keep in mind that for these suites of mods, you'll just want to pick one of them to install as they usually run into conflicts with each other. If you prefer not to install suites though, there are plenty of standalone addons that don't conflict with each other at all.
If you do prefer just using standalone plugins, you can take a look at my picks for best WoW addons .
Addons can actually come in really handy if you're looking to improve your leveling speed as well, and my favorite WoW leveling guides have some great advice on what kinds of mods to install, as well as coming with their own customized map mod that makes leveling fast extremely easy.