Salutations. I am the Thief, and I am quick and quiet. Like a shadow in the night, I stalk my target, and like an arrow, I speed around the battlefield. Be it with bow, blade or bullet, I dispatch my target efficiently, silently and with just a splash of style. 
To those who have not met us yet, you might think of us as outlaws, brigands, or assassins. We are all this and more.  We are swashbucklers, highwaymen, scouts and saboteurs. 
That’s enough of an introduction for now. It is time you actually got to know us properly.  Here, I present to you our tactics, our weapons and our traits, in the hopes that it might better prepare those new to the profession for the event that stands but two days away. Perhaps I’ll even teach the other professionals a thing or two while I’m at it.
The Thief, despite any preconceived notions otherwise, does not need to be solely confined to the role of damaging a single opponent. We can fill many different roles, all of them useful and steeped in tactics.  However, the way we dispatch our foes and act on the battlefield is slightly different from the other professions of Guild Wars 2.
Thieves rely on a mechanic known as Initiative. Unlike every other class, our core attacks (the 1-5 weapon skills) don’t actually have a cooldown. Instead, they cost a specific number of initiative points to use. In the beginning, you’ll have 12 Initiative points, though this total can be increased once you get a little higher in level. Initiative also regenerates slowly over time, and there are a number of skills at your disposal which can give you some initiative when you need it the most.
What this means is that we can string together a series of skills in quick succession without having to wait for a cooldown. This can take a couple of different forms, from spamming the Pistol skill Unloadto giving an enemy a specific series of conditions on the go. If you’d like to learn more about Initiative, you might find this article useful.  
In combat, we function as the single target expert. It is our goal to incapacitate, kill or otherwise neutralize the threats that could kill us or our allies. We can do this in a number of ways, the most obvious being to kill the enemy. 
Thief combat relies on two principles – condition damage and mobility. Many of our attacks can cause Bleeding, and with Venoms at our disposal, we’re able to cause a lot of damage by heaping a small selection of conditions on to our target. However, we are quite fragile, so continuously moving and dodging is of paramount importance.  We are helped in our need for mobility by one of our profession’s mechanics – the Shadow step, which will get you into combat quickly and, some times, back out. Steal, our Profession’s signature mechanic, can be used as a Shadow step, though it can be used quite effectively on its own. In either case, just remember to keep using Steal! It will pay off.
We can also incapacitate our enemies through the use of Stealth. Individually, Stealth can be useful in repositioning or confusing our opponents. While in a group, Stealth can be even more effective as a means of support. Cloaking a number of those people fighting with you in stealth can be an incredibly useful move in combat, and might even save some of their lives.
Stealthy or not, we won’t be much good in a fight without weapons. The Thief has a fairly small selection of weapons to choose from, but once again we function just a bit differently from everyone else.  Our third skill on the weapon bar is determined by the combination of weapons we are using at that time. This is known as our Dual Skill, and can be one of the most powerful abilities in your arsenal if used correctly.
The Weapons of the Thief
Dagger: The Dagger may be our most offensive weapon.   Your basic attacks hit multiple times and inflict weakness, and you can leap to your foe, closing the distance between the two of you very quickly.  Conveniently, the off-hand dagger also gives you a ranged option in the form of Dancing Dagger, and Cloak and Dagger is one of the best ways to acquire stealth.
Sword: The Sword embodies the idea of movement. Infiltrator’s Strike is a phenomenal means of jumping into combat and then jumping out of combat, thus largely avoiding most of the damage in a fight.  In addition, when using a Dagger in the off hand, you have access to Flanking Strike, which is quite superb in avoiding frontal attacks. With a Pistol in your off-hand, instead you’ll be able to Pistol Whip your enemy, which provides you a wonderfully satisfying Daze.
Pistol:  An excellent balance between survivability and damage. Your auto-attack will bleed the opponent, and your second attack inflicts Vulnerability. With an available Daze and area Blind, the pistols can also be quite an excellent support weapon. You’ll also have access my favorite weapons skill, Unload.
Shortbow: The only weapon that doesn’t have access to dual-skills, since it is two-handed. The shortbow gives us a couple of AoE skills – Cluster Bomb and Poison Gas.  This is very nice, considering our overall lack of AoEs.  The Shortbow gives you access to the skill Infiltrator’s Arrow, which is a Shadowstep and one of our class’s best tools for moving around the battlefield. It is, among the Thief community, largely considered mandatory for WvW combat.
Our Traits are another tool for us to use.  Like all professions, we have five different Trait lines to dip our figures into like a coin purse. There are many viable ways to build your traits, though I would strongly suggest investing some points into the Acrobatics Trait line.
Overview of Thief Trait Lines
Deadly Arts: Increases your damage and condition duration.  Many of the traits focus on venom use and conditions in general.  This line has a number of traits that increase the effectiveness of the Steal mechanic, transforming it into a formidable damage-dealing machine if traited properly.
Critical Strikes: Unsurprisingly, this increases your critical hit chance and multiplier. It mainly focuses on increasing your chance of scoring a critical hit, and supplying you with more boons during combat.  It also has a few traits affecting Initiative.
Shadow Arts: This increases armor and healing, and focuses on make Stealth more powerful. This is mainly done through increasing the duration and adding a number of additional effects to stealth, such as healing and boons.
Acrobatics: Increases your maximum health and the duration of the boons you acquire. Most of the traits affect your dodging, either through giving you more opportunities to dodge or reducing the effect of mobility hindering conditions. I consider Feline Grace an essential Trait.
Thievery: Increases your condition damage and reduces the recharge on Steal. Many of the traits turn Steal into a powerful means of shutting down the enemy or supporting your allies. There are also a number of skills to help with initiative management, including the very important Preparedness.

In summary, the Thief is a swift and deadly professional, perfect for those looking for anything from a gunslinger to a skirmisher.   We might not be the toughest combatant, nor the most magically inclined, but I’ll be damned if you can find a better professional with a pair of daggers or a brace of pistols. That is, if you can find us at all.