Difference between different healing classes

I have an alt scholar that i am playing but i have questions. What makes the scholar different then the white mage or astro-whatever?

I am also not sure what the fairies are for. Do they just heal someone on random and if they do then is their a possibility of overhealing and therefore wasting MP

how do i lower the possibility of having 500000 adds come after me, i know that its not good to spam heal cause that agros adds to the healer but should i just focus using DOTS so that im not bringing down my MP so much?

To give you a couple of simple differences...won't get to complicated here.
1. Scholar used more preventative healing with its shields and has really weaker AOE healing (got a bit better in the expansion with Indomitability though). But can do some really good dps. The fairies can be controlled or left to their own (better to control them for serious content). Scholar by far is my favorite of the three healers so far but I have not got my Astrologian to 60 yet so take that with a grain of salt.
2. White Mage is the best pure healer with great single target and AOE healing capabilities. White mages can do great dps also...don't get me wrong.

Biggest difference for scholar is the Fairy. The Fairy will always heal a target when it drops below 20% health, and this always happens, evenif you have it on "obey". The "obey" just prevents the fairy from blowing all of its cooldowns (group cleanse, aoe heal, heal buff, etc) until you actually need them.

The Fairy has no mp, so feel free to have it spam heals to your hearts content. This and aetherflow contribute the lion's share to Scholar's mp efficiency.

White Mage simply has the most powerful heals and the best aoe heals in the game. Watch your MP and you should be fine. They do less DPS than scholar, but they have more damage utility (holy, assize).

Astrologian is like the ginger orphaned stepchild of the healer family. They give buffs and heal. Damage is actually pretty good for all healers, but MP is such a huge issue that few can dps. Overall, there are a lot of difficulties with the AST action economy.

As for managing aggro, be aware of how the enmity table works. Essentially, if a tank charges in to a group, all enemies see the tank as an enemy. Any healing you put out on the tank will add you to their enmity meter. And if your tank isn't doing their job of aoe-ing, some are going to peel on to you.

Usually, I wait until the tank has 3 flashes/overpower/etc off before I start pouring in the heals (1 adlo mid-pull is fine if they've aoe'd. The shield doesn't count as healing.). Always buff your tank with Adlo and Stoneskin before a huge pull (so you can delay healing until he has all the enmity built up) and have your fairy heal so you can split the enmity between you two.