What are AAs?
    AAs are alternate advancement for your Paladin. It is a way to add new skills, spells, abilities and stats to your stock Paladin. Each AA is part of a branch of a tree or line (also sometimes called a path). Purchasing AAs in a line unlocks more powerful AAs further down the line (once a certain number have been purchased; usually 4). Purchasing the last unique ability in a line grants a title prefix for your character (Avenger, Cavalier, Sentry, Knight, Legionnaire)

How do I view my achievements?
    Open up the skills window, which is bound to the "L" key by default.

How do I start earning achievement experience?
    You must be level 20 or above to take advantage of the Achievement system (technically you need to have purchased an expansion from Kingdom of Skies on, but they are bundled free with newer expansions)

How do I earn achievement points?
          Completing non-repeatable quests suited to your character's level
          Defeating special enemies, such as named bosses, for the first time
          Looting rare treasures
          Exploring dangerous locations
          Completing collection quests
          Gain experience at level 70

No for example take the Crusader AA line. There are 5 lines (STR, AGI, WIS, INT, STA) with 5 achievement abilities each, meaning there are a total of 25 upgrades available to you to choose from within a line (26 when you count the initial ability that you must train). Since you only are allowed 70 AA you can only finish two lines (but come close to finishing a third if you like)

In order to train the next available ability in an achievement line, you must have 4 points in the previous achievement ability. The final achievement ability in a line is especially powerful, and requires 2 points to train (and cannot be further improved).

Where do I respec my achievements?

    NPC: Wynia Vethe <Achievement Counsel>
    Location: South Qeynos in the Tower of Three
    Cost: 10GP is standard unless you have changed it recently.

Can I gain achievement experience while Mentoring someone?
    Yes, you will earn achievement experience as if you were the level you are Mentored down to. This works with mobs AND quests.

I heard I can have two sets of AA that I can switch between, How is this possible?
    The item you seek is The Mirror of Reflected Achievements. This crafted item lets you store one achievement profile and switch between another (as you are in your house). It can be crafted by any tradeskiller level 75 or higher with 40,000 faction (267 writs) with their tradeskill society or higher.

    All you need is the Smoldering Reflective Shard which drops in the non-instanced dungeons of Rise of Kunark. It is tradeable and sells for anywhere from 25p to 100p on some servers.