Have you ever tried to describe the rogue class to a friend or family member? I remember when a paladin acquaintance of mine asked what rogues were all about, what their secret sauce was. The question took me off guard, because I had been playing the rogue for so long that I saw everything through rogue-colored glasses. I struggled to find a simple explanation, because we are simply such a diverse class (once you overlook the pure DPS aspect, at least).
While we could be summed up by one word, awesome, I have always felt that describing our class is a real challenge. Once I started thinking seriously about it, trying to find a class-defining moment or example, it hit me. What could be more ubiquitous than a rogue stealth run? An assassination force of shadow, rogues in their most feared form. We approach our opponents unnoticed, we strike from the shadows with great force, and we retreat into darkness before we're ever seen.
Stealth is one of those tried-and-true mechanics, so embedded in the class that we can be confident that it's here to stay. It's not simple, however, and many of the stealth mechanics can be confusing for those who are just starting their rogues. Stealth has seen a lot of changes over the years, and the form we find it in today is far more potent than its humble origins. That doesn't mean that it is without weaknesses, but with clever play and a deep understanding of the ability, you can perform feats that will have the others crying 'nerf'!
What it does
Stealth hides you from your enemy's view, and allows to pass by opponents without being detected. The power of stealth comes from the ability to choose one's own battles. We can avoid killing unnecessary mobs, we can evade those who attempt to kill us, and we can attack at the opportune moment. Without stealth, rogues would suffer a huge blow in their efficiency and survivability. We're a fragile class, because we're designed around not being hit instead of mitigating those hits. Being invisible allows us to reduce the amount of time we're open to being assaulted.
We also have a ton of abilities that can only be used in stealth, called 'openers', and they're core to the rogue class. These abilities set the pace of any encounter: we can choose between stunning the enemy, silencing them, knocking them out, or dealing massive damage. We've also got several talents that allow us to do more damage while stealthed and while leaving stealth, and so managing our stealth status becomes important to maximizing those effects.
Stealth becomes an important part of our survivability in PvP encounters: we can stealth away to regain health, to assault again with another opener, and to avoid powerful attacks your opponent may have ready. The easiest way to re-enter stealth while in combat is to Vanish, though this has a cooldown. The alternative is to leave combat by not attacking or being attacked for 5 seconds. At that point, you can stealth normally. Improved Gouge and Blind provide easy opportunities to restealth, as well as running away with Sprint to break line-of-sight. Any damage we take while stealthed will bring us out, so it isn't really viable to stealth with a DoT effect on you. Use Cloak of Shadows to clear any magic effects, or wait out any bleeds you may have on you at the time before restealthing.
What it doesn't do
Being stealthed doesn't make you immune to anything. You still take normal damage from all attacks, and you are still an eligible target for any AoE ability. The only protection that it grants is the inability to be targeted for single-target abilities, which is powerful in itself. It also reduces our pure mobility, as we move slower than normal while stealthed. This mobility penalty can be overcome by gaining the ability to take a more direct route to somewhere, going between mobs instead of around them. We're still able to be found while stealthed, it's not a magic bullet like Invisibility and every target, big and small, can detect us at some level. We can't just use stealth to escape, either, as without Vanish we can't stealth while in combat.
How detection works
Getting detected is the worst thing that can happen to you if you're trying to remain unseen, and it instantly puts the rogue at a huge disadvantage. Being detected while stealthed is a big deal: mobs will aggro to us, and players will be able to target us normally again, as well as giving away our location. We can avoid all detection by staying behind our enemies, although every player has a small sphere of detection with about a 2 yard diameter. If you're in that 2 yard sphere, on any side, you'll be detected. You're also at risk of detection if you're directly in front of your enemy, and at a smaller risk of detection if you're in their peripheral vision. Enemies that are higher level than you have a higher-than-normal chance to detect you, like bosses or elites. Enemies that are below your level have a less-than-normal chance to detect you, though the 2 yard sphere will cause you to be detected no matter what. Once you're behind them, though, you're completely invisible. See this diagram for details:
If you're directly in front of your target, you can actually get pretty close before being detected. Here's a picture of myself with the Master of Deception talent, approaching a priest from the front. I stopped the moment I was detected, and took a half-step back. You can see that even from the front, you can get pretty close to your opponent without even the hint that you're there.
Without MoD, it's pretty obvious that you get detected more often. While MoD is traditionally considered a PvP-only talent, it is mandatory for any stealth work you intend to be doing. It's also very useful for sneaking past higher level mobs, who have very strong passive detection and will see you from further away than usual. Here's an example distance without MoD:
If you absolutely much approach your enemy from the front, you can use a preemptive Vanish to get there successfully. Vanish greatly improves the power of your stealth for 10 seconds, allowing you to pretty much sit on top of the enemy without being seen. You can use Vanish while stealthed, so think of it as a temporary 'stealth boost cooldown'. Here's an example of how close you can get while having the Vanish buff active:
As I mentioned earlier, you can be right behind your opponent without them ever knowing. The trick is to avoid stepping into the 2 yard detection sphere, and since we can attack from 5 yards away, there's no real reason for us to get that close anyway. Many rogues make this mistake: they'll move in for Sap and get too close, allowing the target to detect them. You remain detected even if your opponent is Sapped, and so if they were to use a PvP trinket or get out of that Sap, you're now temporarily detected and attackable.
Learning the exact distances at which you will be detected is a largely trial-and-error endeavor, the easiest way to learn is to simply stealth as often as possible, and test different approaches. You'll get caught, quite a bit at first, but you will learn how to approach with grace. There's nothing worse than being caught while stealthed, so learning to avoid that will ensure that you get the opening advantage that you deserve.
Ways to be detected
Hunters are really the bane of any sneaky rogue. With their Track Hidden ability boosting their passive detection and their Flare which removes stealth from anyone in a certain area, you'll want to be extra cautious to avoid letting a hunter find you in the open. Nearly every other class has an AoE ability they can use to draw us out of stealth, including non-damaging attacks like Demoralizing Shout. You can try to time your approach between waves of the AoE, but it is typically easier to wait out your opponent's patience before engaging. The mind game of trying to bring a rogue out of stealth is a huge part of the PvP game, and learning to approach your target undetected will be key to winning the majority of your battles. In a rogue vs rogue duel, the opener is literally the only important part of the duel: 90% of rogue-rogue duels are won by the aggressor.
Special mobs are also able to detect you, as seen by the 'stealth eye' detector over their head. Blizzard threw a few of these guys into the major cities to prevent us from completely dominating unsuspecting townsfolk, and they also added them to particular areas in dungeons that they don't want rogues bypassing. These guys will see you as if you weren't even stealthed, but they can be tricked in one way. They still can't detect you from behind them, and so with clever use of Distract to turn them and a quick Sap to incapacitate them, you'll be able to get past them. It's not an easy trick, but it is possible.