Add-ons are everywhere in World of Warcraft. Whatever level you're on, you need to have one. Want to sell some goods? You'll need the Auctioneer add-on, unless you enjoy being ripped off for all your hard work. And of course you'll need to find items to sell. The only way to do this efficiently is with the Gatherer add-on. There's a great amount of add-ons available, and experienced players will have already downloaded dozens of add-ons before reaching level 70.
An add-on is a type of software, or modification to the software. Since WoW is a massive multiplayer online role playing game (MMORPG) connected with the internet, the updates are readily available and kept up to date. Keep all of your add-ons in a practical place on your computer. You'll need quick access just in case you need to delete an obsolete add-on or replace it with another.
The perfect add-on is one that boosts your ability as an all-around player. Still experienced players know that not every add-on is helpful to all players at all times. The Auctioneer, Gatherer and Quest Helper add-ons are essential for everyone, but a few add-ons are created exclusively for specific races, classes, and professions. Keep this in mind as you go down the list of add-ons posted regularly to WoW sites and hosted add-on pages. Also, be advised that some add-ons work best for players at a certain skill level. They can even be detrimental for lower level players.
As always, make sure to download from a reputable site. Otherwise you compromise your security and risk being banned by Blizzard, WoW's creator.