Non-Gamer's Guide to This Post
A raid is a large group of people banding together to fight a hard-to-kill boss. You can only bring a maximum of 10 people to some raids and 25 to others, depending on boss difficulty settings.
Healers keep people from losing health and dying.
Tanks have low damage and high defense. He or she keeps the boss's attention (ie, aggro) so that it doesn't attack the rest of the group.
Dps (damage-per-second) attack the boss and put out a lot of damage to kill it quickly.
This one tip will get you into raids a lot more than you otherwise would. It's very simple, takes some time, but is very effective.
Roll a healer.
The idea is to be willing to fill a slot that your guild is missing. If you're low on healers, going heals is a surefire way to get in and see raid instances.
Dps, on the other hand, is the easiest and most common spec, a dime a dozen, and even being good dps doesn't guarantee anyone a raid slot.
Healing takes a lot of the competition for raid slots right out. I would say tanking does the same, but not so much with the death knight craze.
Tanks and heals are essential to any instance or raid. If you perform one of these essential roles and aren't a nincompoop, you will be taken more often than if you just played dps.
"I don't enjoy healing! Or tanking! Dps is fun!" So sayeth 50 other people in your guild. Good luck beating them for the raid spot.
I read a WoW Insider post and thought about how tough it is for my guild's raid leaders to choose a team. There are so many people signed, they always go back to basics and look at gear and previous performance. A lot of people are pretty equal in skill, so they get rotated per raid based on gear and who's best for the boss, and others are seasoned veterans who would get us where we needed to go if we were that kind of guild, but it usually just comes back to "How ready are you and how well do you fit what we need this raid?"
I'm not saying everyone must go heals, because some people hate heals, but before you start getting upset that you weren't chosen for this or that raid, consider how many dps your guild has (how many of your class your guild has!) and who you're competing with for a spot.
If you've worked hard and tried your best and didn't get rewarded for it this time, put the raid into perspective, calm down, and work on your gear. Patience. You'll get in. It often just takes longer if you're dps.