Mods And Add-ons

Mods and Addons have a very, very special role in adjusting the user interface (UI) to players’ specific needs or preferences while playing the game. Mods are an abbreviated form for modifications. In World of Warcraft, they act like a third-party addition that helps players greatly change or improve some aspects of the World of Warcraft’s user interface.

Mods, sometimes called User Interface Modifications (UI Mods), in general, can do a wide diversity of main tasks in the gameplay. They can be used to add a set of new buttons to the player’s screen, change the built-in player and party frames or automate some aspects of spellcasting as well.
In certain cases, mods can totally be a tool that is supplementary to the standard World of Warcraft user interface that generally provides users with much more advanced and varied functionality.

AddOns are exactly the same as Mods, the only major difference is just in the matter of their names. Normally “Mods” tend to mean smaller things that merely modify existing functionality of the user interface, while AddOns are more likely to add extra functionality to the user interface. 

The most well-known site for Mods and Add-ons is probably the curse network.