The purpose of an add-on, or mod, is to make playing easier. As WoWWiki puts it, “An AddOn is just some files you can put in your game folder that can (theoretically) improve your interaction with the World of Warcraft game (i.e. make it easier to play, or give you more information about what’s going on in the game).” WoWWiki has a more detailed explanation here.
What I love about Curse is that you get what you need in just a few clicks. Although their site contains thousands of different add-ons for just World of Warcraft alone, you don’t necessarily need to browse all of them to find which ones are right for you. The most downloaded add-ons are always listed first, along with how many people have downloaded them and the date it was updated, (you want to make sure it was updated since the last patch). So just because a bunch of people are downloading them, does that mean it’s the right one for you? Well, there’s gotta be a reason why a shitload of people are using that particular add-on over the others, right?? Curse.com contains add-ons for other games, too, but the link above will take you straight to the World of Warcraft add-ons.
Pretty much does what Curse.com can do for you, but not as pretty. The most popular add-ons are marked with a star.