What is a raid?
A Raid is when players kill specific mobs to obtain items which cannot be gotten anywhere else. Usually such mobs don't give much xp, but have some special behavior patterns (nuke, spawn adds, etc..).
How is a raid run?
Practically each raid is unique. They are handled differently depending on mob, faction, server, available raid force and even raid leaders (read on for explanation of terminology). But they have several similarities that ease communication and assigning of tasks.
If the raid force consists of one team, then communication is usually carried over the team channel. For additional help or options a raid bot can be used for this purpose too. A raid bot is a specifically set up channel which raid participants are joined to. Raid bots usually have a set of rules and instructions in their help files which are accessible by its members in game.
If the raid consists of one guild, communication is usually carried over organization channel or even more frequently over a raid bot, since distributing loot is a bit tricky is there is more than just one team.
If the raid consists of several teams, of which members belong to different organizations, communication is carried over a raid bot. This bot can be dedicated to raiding just one (type) mob or some more general one.
Raid force classification by role:
Tank: Tanks usually are high hit point, high armour class participants (commonly enforcers, keepers and soldiers). Their primary task is to hold aggro from mob(s) to ease work for healers.
Healers: Healing is usually undertaken by doctors, although indeed any profession with heals can be used. Their primary task is to keep tank(s) alive.
Crowd Control: Ability to focus the raid force attention on as few mobs at same time as possible, this class consists mainly from calmers and pullers. They complete their tasks by first calming adds and secondly by pulling mobs without adds.
Damage Dealers: This consists of everyone else in the raid force. Their primary task is to to kill without pulling aggro.
Depending on the raid and size of the raid force, several of these classes can be assigned, but usually it's just one Tank and Crowd Control. Healers tend to be evenly distributed through raid teams and while the healer in the Tank's team watches over him/her, other healers assist if needed. One player can take several roles if needed.
Raid force classification by tasks:
Raid Leader: This is the Top Dog, who assigns roles and gives commands to the participants. The raid leader starts the raid and completes tasks to ensure that the raid is successful.
Caller: The caller is someone who knows mobs that will be encountered and assigns targets on which the raid force should concentrate.
Loot Distributor: This player handles loot. Their job is usually only in effect only in high activity raids, otherwise Free For All condititions are set into action.
Raid force classification by dedicated teams:
Tank Team: This team consists of a tank, at least one healer and several professions that ease work for those two roles (professions with shields, evade, nano buffs, etc..).
Kill Team: This team consists of high damage professions which is optimised by additional professions that can significantly boost damage to those in their team.
Heal Team: When encountering mobs that deal lots of damage without area of effect, making a team with doctors helps to synchronize heals.
So why such classifications?
This was developed by trial and error. First of all you need to remember that while you're teaming with someone, he might be located thousands of kilometers away from you. Assigning specific roles and establishing "chain of command" helps to battle against lag, cultural differences and helps coordinating attack. If everyone knows his task, a raid runs more smoothly and is less likely to fail.
:: When joining a specific raid for first time ask about rules.
:: When joining a specific raid for first time ask if mob has any specific behaviour (nukes, runs away, spawn adds, etc).
:: When joining a specific raid for first time ask about loot distribution (if you don't agree it's better that you leave at start than in the middle or end of the raid).
:: Always follow raid chat and commands.
:: Do /macro assist /assist "name" (substitute "name" with caller's name).
:: Always call back pets when mobs is dead (provided that pets are even allowed at raid).
:: Buff anyone that asks for buff.
:: Familiarize to potential loot prior to raid, especially what you can use and what you cannot and which items are NoDrop.
:: Bring enough ammunition, first aid/nano stims and packs to last you through the raid.
:: Don't do any solo moves. Let more experienced players guide you through (either by actions or advice).
:: Don't attack mobs at random, your contribution to the raid will most likely be wasted.
:: Don't attack mobs half a zone/instance away, you'll bring adds.
:: Don't use AoE nanos, be that nukes, auras or perks.
:: Don't go /afk in middle of raid without agreement from other participants.
:: Don't run back to safety, after pulling mobs to the raid force, you'll just bring adds. Go back to the raid after you have died.
:: Don't buff anyone without being asked, that player might be low on NCU and won't be able to execute nanos when needed.
:: Don't loot items you're not entitled to or are non-drop which was not assigned to you.
:: Don't hold open mobs loot to examine what it dropped or when someone else got loot rights.
These are some general rules and tips that apply to most raids. But like I said, with different mobs, servers and factions, not all of these rules apply, some use different tactics. Never be afraid to ask about certain aspects of the raid when joining it for first time.