I only subscribed about 2 days ago after coming back to WoW after 6 years. The game has changed so much since I left right around the Cata release. I decided to start fresh on a new server. I still had all my old links and logins to places like Curse, WowInterface, etc. Loaded up all the addons I had favorited years ago (of the maybe 1/3rd that were still maintained). Got to level 20 in a few hours of playing. Had to look a bunch of stuff up.
I used Zygor for legion and BFA, used AA on an alt for BFA. Personally i like Zygors format more. I like being able to target things like achievements. I used Zygor to get my Mindworm and my Lucid Nightmare as well as finish the Rep grinds for the allied races. I have zero complaints. My experience was great. I also used Zygor for some professions, recipe acquisition, a couple mounts, all of the Suramar quest line. Thank God for that. And for the last zone of Draenor on my Demon Hunter.. i forget the name.. Whatever the big daily hub thing was called.
The part you need to understand too, is that the Zygor addon doesn't use an App like TSM to update many times per day your auction value. When i bought the gold guide i remember that i didn't understand much either, the results were random and from items i can't make much gold with. It just scans the auction house based on their most recent value update which can be days and days ago.
AddOns (sometimes called addons, add-ons, or mods) are collections of Lua/XML code that augment, modify, or replace World of Warcraft's default user interface. Addons vary greatly in scope and size — they may display information not presented at all by the default UI, add functionality to the default UI (e.g. additional buttons you can press to do things), modify the default UI's appearance (e.g. by changing colors or positions of UI elements) without changing its functionality, or even replace the default UI entirely.
Some addons are standalone, and are able to run without any additional addons being installed; others can require certain other addons to be installed to work at all, or integrate with other addons to provide additional functionality. Collections of addons that have been put together to form a coherent user interface are sometimes referred to as UIs.
Often the name modification (or "mod" for short) is used interchangeably with AddOn. More specifically, a mod is a third-party AddOn that alters or enhances some aspect of the game's interface. Mods can perform a wide variety of tasks, including adding new buttons to the screen, altering the built-in player and party frames. In extreme cases, mods completely supplant the standard game's interface, usually offering more advanced functionality.