Best answer is to start with the FAQ in the JOB Class area for BLM (linked and quoted below).
FAQ site:
http://ffxi.allakhazam.com/db/jobs.html?fjob=3&mid=108742636464330556&num=112
Short answer as quoted from that faq:
--- Elemental debuffs ---
This is a common area of confusion for startup black mages. It's easy to pass over the elemental debuffs because they do minimal damage for the 25 MP they cost. However, it's the stat debuff associated with each one that should draw your attention.
Here are the spells.
Shock - MND debuff, lightning damage (decreases magic resistance)
Rasp - DEX debuff, earth damage (decreases melee accuracy)
Choke - VIT debuff, wind damage (increases non-magical damage taken)
Frost - AGI debuff, ice damage (decreases evasion)
Burn - INT debuff, fire damage (increases magical damage taken)
Drown - STR debuff, water damage (decreases melee damage dealt)
Bear in mind, the chances of these spells landing is based largely on the resistance of the mob.
Not all of these spells can be stacked with each other. To understand how this works, you need to revisit the elemental circle. A spell cannot co-exist with a spell that is immediately adjacent on the circle (i.e. You can never have Shock and Drown on a mob at the same time). If you try, you will either get no effect (i.e. Frost cannot be applied if Burn is active), or you will overwrite the current spell (i.e. a successful Frost cast will always overwrite Choke). Again, look at the elemental circle to understand why this is the case. You will never be able to have more than 3 of these spells active on a given mob at one time. If you want to lay down as many debuffs as you can at once, you have to choose between a Shock-Burn-Choke combo, or a Rasp-Drown-Frost combo. Don't feel like you have to have three going for the entire duration of the fight, or you will waste time that could be spent resting, because you are trying to get a debuff to stick that might not even make a difference in the outcome of the battle.
In the majority of cases, the only spell you need to worry about is Frost. I'm not saying the others don't have their uses, just that Frost will give you the biggest bang for your buck. By lowering the mob's AGI, the melees in your party will hit more often, thus doing more damage, building TP faster, and allowing you to magic burst more frequently. It's a win-win-win situation.
**
YOUR SPELLS AND YOU:
=================================
As a mage, your spells are your lifeline. Knowing how to use them effectively is what will set you apart from the rest. Here are the things you need to know to make this happen.
--- The elemental circle ---
Memorize this circle now, if you haven't already. It affects so much of BLM life, I can't even describe it all in this guide.
-> indicates a strength
Fire -> Ice -> Wind -> Earth -> Thunder -> Water -> Fire -> Ice -> etc.
Light <-> Dark
Each element is strong to one, and weak to another. Light and dark have a unique relationship in that they are each vulnerable to the other.
I will refer to this circle many times throughout the rest of this guide. Learn it, love it, live it.
As a side note, when fighting elementals, and even the avatars at higher levels, quite often the element that it is weak to is the only one you can use for anything more than a full resist. Save yourself the time of finding this out for yourself and just use what you know will work.
Unquote.
Then read the Rdm job faq:
http://ffxi.allakhazam.com/db/jobs.html?fjob=2&mid=1098565860385835838&num=40
And finally the Whm job faq:
http://ffxi.allakhazam.com/db/jobs.html?fjob=1&mid=1099270238991277399&num=18
Edited, Sat Jan 1 16:35:12 2005 by Jaxxsword