Another in my 'posts for the newbies' collection for my guild. We have a lot of players who are new to the game, and I am sure some of them didn't know this.
Now, I know when I first started I hadn't got a clue about threat or agro, and I suffered because of it. These days I generally suffer because other people don't know about it, but that's a side issue.
Basics of Threat
How much Threat you have is indicative of how much a given mob hates you and wants you to die. If you have the most threat then you will generally be the one who the mob is taking out all of their childhood issues on. The tank of the group is constantly trying to be on to of the threat list, the rest of the group should be sailing as close as they can while staying behind the tank.
Because threat is measureable there are addons to help you gauge how much threat you are causing. Omen is a very good threat meter, and can be found on http://files.wowace.com/ which also contains a long list of other, normally quite good, addons. Omen is good because it will flash your screen when you pass 90% of the tank's threat, and a good visual indication like this is better than a quiet little bar creeping upwards.
The more people in a group that have Omen, the better. Omen doesn't monitor other people's threat unless they have it, so if everyone has it then everyone benefits. If only the tank and you have it then it's still good because you can stay below them and let the other members of the group worry about getting themselves killed.
On a very basic level, threat is accumilated as follows:
1 point of damage = 1 point of threat
2 points of healing = 1 point of threat
Talents, items, enchants and gems can all affect this rate of threat generation. Tanks generally want to up their threat generation while healers and DPS classes want theirs to be as low as possible.
Basics of Agro
Threat and agro are two different things, although they are often interchangably used. One "generates threat" but one "has agro" or "takes agro". Having agro means you are the one the mob is mashing or chasing around and trying to mash.
When the pull is first made is the time when agro is easiest to take, as the tank will not have much threat with the mob(s). If you're a healer and the tank fires his bow at a mob who is in a pack of three, then anything you do will cause you to take the agro of the mobs the tank didn't shoot. This is because the tank has zero threat with those mobs. They will run to him because he is provisionally top of their threat table, but any action which causes global threat (buffing, gaining rage, healing, etc) will pull the agro of these mobs.
Once the tank has started to generate threat with the mobs then you need to exceed the threat he has caused in order to pull agro. For example, (assuming no threat mitigation for either tank or healer) if the tank has hit the mob for 300 hit points with his bow and you heal him for 780 hit points (divided by 2 because it's healing threat) before he does anything else, then you will pull the agro off of him. This is because, as a ranged class, you need 130% of the tanks threat in order to pull agro. Melee classes only need 110%. This means that if you're a rogue and you smack the mob for 330 hit points as it runs past you on its way to the tank, then you will have agro. Congratulations, I just hope red works with your current gear combination.
Only healing which actually replenishes hit points counts for threat purposes, so if the tank was only missing 100 hit points in the above example, then you'd not have pulled agro, because although you hit him with 780 hit points of healing, he only took 100 (50 threat) points worth.
What does all this mean? Try to think about the threat you are generating with specific actions. If you pull agro then the tank has to have 110% of your threat before he gets the agro back. If you've pulled agro by opening with pyroblast, this might mean you need the tank to growl/taunt the threat off of you because you're so many points above him on the table. If the tank growls/taunts the threat off of you and the healer then pulls you could have caused a wipe, because the tank now can't growl/taunt the threat off of the healer because the ability is on cooldown, meaning the tank is going to have to exceed the healer's threat, and the healer is still going to be generating threat by healing herself...
It also means that if you've pulled agro the safest thing to do is to stop DPSing and let the tank take the agro back. If you run from the tank then they can't catch the mob and thus can't take the agro. In instances mobs generally run faster than players which means that while you and the tank are keeping pace with each other, the mob is pounding you a new one. If you're hitting the mob with instant cast spells as well? Well, you clearly want that threat too badly, and who am I, as a tank, to argue with that? Also, chasing you around trying to take the mob off of you means I'm not generating threat with the other mobs, and they're now on the healer, who's having to heal you, and me.
In short, threat is what grouping is all about. Knowing how much you're generating and knowing how much the tank is generating. If you're generating it to the point where you're taking agro, then slow down your DPS. It's far better for you to throttle your DPS than for the group to deal with the chaos of your over-agroing.