Before we can kill any monster, we need to bring them to the raid. This is the job of the puller and their team.
The puller is usually a monk, shadowknight, bard or even a necromancer - they all can lose aggro should they need to. And they often do. It is thus important not to cast any spells on them when they are pulling or just looking around, because such a spell would immediately aggro all the monsters and bring them to the raid, causing a lot of problems. If in doubt whether or not a spell (like a heal or buff) can be cast on the puller, you should ask them, rather than to assume that it's OK. The same is true for bard songs; bards will need to turn all AE songs off during pulling.
When waiting for the monsters to come, the tanks should stay in the front and everyone else should be behind them. This is to ensure that the monsters will jump on the tanks first and not on the healers or the casters. Tanks will have their tanking order set, so often just the main tank and the offtanks need to be in front, all the other melees should stay in the back as well until assist is called. When the camp is clear, everyone should return to their position behind the tanks and wait for the next pull. Sometimes pulling a monster takes a long time, so please be patient if that's the case. Do not sit down when a pull is in progress; monsters like to jump on people who are sitting down (because sitting people are more vulnerable).
Noone should ever interact with pulling in any way, unless they are asked to do so by the puller or the raid leader. The puller knows what they are doing and why they are doing it and even a simple thing - like moving too close to the puller to see what's happening - can have catastrophic results, like a train of monsters wiping the raid.
When possible, we may choose to give tanks their targets that they should aggro on a pull before the monsters make it to the camp. This technique is called pretargetting. Before the puller brings the monsters, someone (usually the puller or a rogue) will ask each melee to assist them. When all monsters that will likely make it to the camp on the pull are targeted by a melee (or a mezzer), the puller brings them. This makes it much easier (and faster) to get the monsters under control, because the melees (and mezzers) do not have to look for a monster they should take care of. Some raids (especially Vex Thal) depend heavily on pretargeting and are almost impossible to do without it.
Sometimes we will not be pulling monsters to us, but instead moving towards them; if that happens, it will be clearly mentioned.