See the images for a quick look at the ingame UI! This might give you a better idea of what is added to the default Blizzard UI.

Achievement Filter completely replaces the default Achievements tab and adds several new tabs that order the achievements by expansion, pvp, events or other curated lists. Over 6400 achievements are added so far spread over 1200+ categories, linked to 1800+ zones and 120+ events.

Now also mostly available in German, French, Portuguese (Brazilian), Russian and Chinese (Simplified)!

Tabs

Achievements: A copy of the default Achievements tab but with all the additional features.
Expansions: A collection of all the achievements in game sorted per expansion. See Categories for more details on this tab.
Events: All ingame event related achievements can be found here.
Player vs. Player: A PvP'ers dream with all achievements earnable trough PvP.
Specials: A combination of curated lists that might appeal to certain achievement hunters.

Categories (Expansions)

The Current Zone category shows which achievements are related to the current zone you're in.
The Selected Zone category shows which achievements are related to the zone you're viewing in the World Map.
The Watch List category shows achievements you're actively tracking. Showing this category can be toggled in the options.
The Tracking Achievements category shows tracking achievements usualy not visible. Showing this category can be toggled in the options.
The Excluded category shows excluded achievements you don't want to see. Showing this category can be toggled in the options.
Each expansion has its own category: Classic, The Burning Crusade, Wrath of the Lich King, Cataclysm, Mists of Pandaria, Warlords of Dreanor, Legion, Battle for Azeroth, Shadowlands, Dragonflight and The War Within.
Each expansion is divided in Zones, Dungeons and Raids and Pet Battles where available.
Wrath of the Lich King has separate categories for 10 and 25 man Raids.
Wrath of the Lich King expansion feature: Argent Tournament.
Cataclysm expansion feature: Tol Barad.
Mists of Pandaria expansion feature: Brawler's Guild, Scenarios, Proving Grounds.
Warlords of Dreanor expansion feature: Brawler's Guild and Garrison.
Legion expansion feature: Brawler's Guild and Class Hall.
Battle for Azeroth expansion feature: Brawler's Guild, Island Expeditions, War Effort, Heart of Azeroth and Visions of N'Zoth.
Shadowlands expansion feature: Torghast and Covenant Sanctums.
Dragonflight expansion feature: Dragonriding, Dragonriding Racing and Primal Storms.
The War Within expansion feature: Skyriding Races and Delves.
Unobtainable Legacy categories can be found when the 'Not Obtainable' filter is checked.

Features

Enhanced searching, filtering, sorting and tracking of achievements per tab.
Reward filters to show only achievements that award mounts, pets, titles, toys, transmog, tabards, teleports, and housing decor.
Enhanced achievement tooltip to show more about the achievement and related achievements.
Enhanced unit and item tooltip to show if the unit or item is required for an achievement. (Still working on expanding this.)
Movable windows with the option to remember their last position.
Achievements are sorted in a way so they make more sense and are easier to find.
A Right Click Menu for each achievement with a link to Wowhead, IAT (3.18.0+) (if tactics are available), Transmog sets (if linked to collecting it), Go to (related achievements) and wow-petguide.com (Xu-Fu's Pet Guides).
A tutorial for first time users with an ingame explanation of the most important features.
The option to keybind the new tabs so it opens in one go. (Game Menu -> Key Bindings -> Addons -> Krowi's Achievement Filter)
ElvUI and GW2 UI skin. (See the ingame options on how to enable each part.)
Clicking achievements in meta achievements select the achievement.
Hovering achievements in meta achievements shows more information about the achievement.
World Map Button with its own Selected Zone category to show achievements for the zone you're viewing.
Watch List to track achievements you're actively working on, with options to display with or without subcategories.
Exclude achievements you're not interested in.
Event reminder for active events with achievements with a compact version option.
Compact achievements similar to the summary achievements.
Overview of which character earned how many achievement points, with per-character progress tracking.
Saturate achievements based on earned by your account, current character or a mix.
Custom objectives for certain achievements. (If you find missing ones, let me know.)
A summary view for each new tab.
A new calendar view that puts achievements earned on their respective day.
A data manager to manipulate your saved variables.
An open API for other developers to add additional categories with achievements.

More features can be requested via the issues system or discord.

Feel free to join discord and say hi: https://discord.gg/mdBFQJYeQZ.

Creating and maintaining addons takes a lot of time. If you can miss something, feel free to drop a donation!

Krowi's Achievement Filter