Raiding has always been part of the game, but it has changed significantly over the years, as a result, there a number of parts of the game very unique to raiding.
This is focused on how raiding works now, and what is different between solo/small group content and large group content of raids.
Group Size: Normal and Heroic use the Flex Raid system where you can have between 10 and 30 people in a raid where it will re-scale to your group size. This is an important feature that allows groups to adjust on the fly to accommodate new members or others leaving as the raid goes on.
Cooldowns: Many cooldowns work differently in raids, Tranquility and Divine Hymn Heal for less, Battle Res is on a shared timer (this includes Soul Stone, but not Reincarnation), also group-wide powerful abilities need to be coordinated by the raid leader; Blood Lust is the most common but also buffs like Commanding Shout are often called by the raid leader.
Another twist with raid wide cooldowns, they are also reset upon engaging with a boss, so if there is a specifically hard trash pack the Raid Leader might call to reset the boss. If this happens make sure you hit the boss with an ability to get your cooldowns back; and try not to die.
Loot Systems: There are several loot systems, some groups use Master Loot, others use Personal Loot. For your first raid, you likely want to find a group using Personal Loot if you can, but a Master Loot group with your Guild or a Pick-Up Group to learn the Raid is a viable option.
Loot Lockouts: You can loot each boss once a week (resets once a week) per difficulty, but you can be in a boss fight multiple times on the same character. You will just be “Loot Locked” any won’t get loot from the boss, and are not eligible to be traded loot from that boss from other players.
Bonus Roll: I will go into this in more detail in a later section, but coins are an important part of getting loot, as you can use them to re-roll for a loot drop off a boss for a chance at gear regardless of the loot system. This is loot that is locked to you and cannot be traded so you will want to save them for bosses where you need the most gear.
An important note is that you can Bonus Roll the same boss and difficulty multiple times a week to try to ‘target’ a piece of gear.
Tier: This is going away in Battle for Azeroth (BfA), but Raids are where you get Tier which provides powerful and unique bonuses for your character.