Let's pretend you're a Mesmer in Guild Wars 2 for a moment. As you spy your prey coming over the ridge of that mountain, you ready your greatsword for combat, as well as your hands, which are tingling to cast purplish beams of death at anyone who opposes you. You're a battle-mage, and you're not scared of anything.
As your foe comes over the mountain, clearly targeting you, you just smirk as you instantly create two clones of yourself who unleash their wrath upon your enemy, via beams of light and translucent swords. It won't matter if the enemy hurts them, because as soon as your clones take the enemy out, they'll shatter into butterflies.
The way of the Mesmer, Guild Wars 2's final profession, is the way of illusion and confusion. While not dealing tons of straight on damage like a Warrior or Ranger, the Mesmer's role in GW2 is to be the master of utility. They can buff their allies all while placing terrible, negative conditions on enemies. Mesmers can change the face of any battle, and a Mesmer who is efficient at using their clones and phantasms can completely destroy enemies in both the PvE and PvP environments.
Our hands-on time with the Mesmer was intimidating at first, but once you realize how amazing the Mesmer's Illusions can be, the profession truly spread its (butterfly) wings. Illusions are created by a Mesmer for specific purposes, and anyone can see them and attack them. Mesmers can spawn up to three Illusions at once, but the Illusions have limited HP, and they die as soon as their target dies. The two types of Illusions are Clones and Phantasms.
Clones look just like your Mesmer in appearance, but they have low health and don't do much damage. Some of the Mesmer's utility skills, like Mirror Images, allow you to spawn two clones to attack your foes for you. However, most moves that summon clones are utility moves that will give you a leg up in battle. Leap is such an example.
When your Mesmer uses Leap, you spawn a clone who leaps to the target and cripples them. You then have five seconds to swap with your clone and attack the target. Leap is a fantastic PvP move, but it's also great to use to catch up with an enemy who's running away from you.
Phantasms are the Mesmer's more powerful illusions. They look like the caster but they have their own weapons and they act a lot differently than clones. I found myself constantly using the Phantasmal Berserker skill, which summoned a Phantasm who used a devastating whirling attack that damaged and crippled any enemies near me.
As stated earlier, you can have three illusions up at a time, and once they kill your target they disappear until you resummon them. If you want to be more strategic with your Mesmer, and you know you're going to defeat an enemy soon (thus losing your Illusions), you can choose to destroy your illusions early by Shattering them. Shattering your illusions will destroy them and place negative secondary effects like stun or confusion on nearby enemies.
Personally, I feel like the Mesmer will be one of the more difficult Professions to master in GW2. Those who want to play the Mesmer at the professions' maximum potential are going to require a lot more skill than those who might choose other, more simplistic professions like a Warrior. With the Mesmer, it's critical to constantly keep track of your Illusions, like which ones you want to Shatter on which enemy, and which ones you don't. There are also so many other Mesmer utility abilities, like ones that stealth the whole team, or make everyone run faster. Essentially, if a skilled Mesmer shows up at the right time, they can really turn the tide of any fight.
My favorite part of playing the Mesmer in PvP was running around with my clones and absolutely destroying people then quickly changing over to more of a support role in order to help my team defend points or to help them get to points faster. Ultimately, the Mesmer is a micro-based battle mage, and if you want to okay a hybrid Profession who can support or deal damage, then you've found your match with the Mesmer.